不废话，直接上代码

前端代码

<input type="button" class="layui-btn" value="Test-GetFileFromWebApi" onclick="GetFileFromWebApi(this)" />

<script>
    function GetFileFromWebApi() {
        location.href = '/api/WorkOrderAPI/GetFileFromWebApi';
    }
</script>

接口代码

        /// <summary>
        /// 从WebAPI下载文件
        /// </summary>
        /// <returns></returns>
        [HttpGet]
        [AllowAnonymous]
        public IHttpActionResult GetFileFromWebApi()
        {
            string filePath = string.Empty;
            var browser = String.Empty;
            if (System.Web.HttpContext.Current.Request.UserAgent != null)
            {
                browser = System.Web.HttpContext.Current.Request.UserAgent.ToUpper();
            }            

            string excelFile = string.Empty, DownloadExportPath = "DownloadExport\\";  DataTable dt = new DataTable();//这里根据实际逻辑赋值 KP.Commom.OperationExcel _excel = new KP.Commom.OperationExcel(); string s = _excel.DownloadExport(dt, System.Web.HttpContext.Current.Request.PhysicalApplicationPath + DownloadExportPath + "系统客户.xls", "系统客户", out excelFile); filePath = System.IO.Path.Combine(AppDomain.CurrentDomain.BaseDirectory, DownloadExportPath, excelFile);

            HttpResponseMessage httpResponseMessage = new HttpResponseMessage(HttpStatusCode.OK);
            System.IO.FileStream fileStream = System.IO.File.OpenRead(filePath);
            httpResponseMessage.Content = new StreamContent(fileStream);
            httpResponseMessage.Content.Headers.ContentType = new System.Net.Http.Headers.MediaTypeHeaderValue("application/octet-stream");
            httpResponseMessage.Content.Headers.ContentDisposition = new System.Net.Http.Headers.ContentDispositionHeaderValue("attachment")
            {
                FileName = browser.Contains("FIREFOX") ? System.IO.Path.GetFileName(filePath) : System.Web.HttpUtility.UrlEncode(System.IO.Path.GetFileName(filePath))
            };
            return ResponseMessage(httpResponseMessage);
        }         

公用方法

        /// <summary>
        /// DataTable转换Excel并下载到本地“下载”文件夹里
        /// </summary>
        /// <param name="data">DataTable</param>
        /// <param name="sheetName">工作簿名称</param>
        /// <returns></returns>
        public string DownloadExport(DataTable data, string filePath, string sheetName, out string excelFile)
        {
            string rMsg = string.Empty;
            string _excelFile = string.Empty;
            try
            {
                _excelFile = Path.GetFileName(filePath); //文件名
                string strExtenName = string.Empty;

                //检测是否存在文件夹，若不存在就建立个文件夹 
                string directoryName = Path.GetDirectoryName(filePath);
                if (!Directory.Exists(directoryName))
                {
                    Directory.CreateDirectory(directoryName);
                }

                //判断文件是否存在
                if (File.Exists(filePath))
                {
                    string strFilePath = Path.GetDirectoryName(filePath);
                    strExtenName = Path.GetExtension(filePath);
                    string fullFileName = Path.GetFileName(filePath);
                    string strSubName = fullFileName.Replace(strExtenName, "");
                    //获取当前目录与当前文件同类的所有文件集
                    string[] hasFileList = Directory.GetFiles(strFilePath, strSubName + "*" + strExtenName, SearchOption.AllDirectories);
                    if (hasFileList.LongLength > 0)
                    {
                        int fileSort = 1;
                        string strFileName = string.Empty;
                        foreach (string item in hasFileList)
                        {
                            string loopFullFileName = Path.GetFileName(item);
                            strFileName = loopFullFileName.Replace(strExtenName, "");
                            int strLen = strFileName.Length;
                            string strleft = strFileName.Substring(strLen - 3, 1);
                            string strright = strFileName.Substring(strLen - 1, 1);
                            int leftIndex = strFileName.IndexOf(strleft);
                            int rightIndex = strFileName.IndexOf(strright);
                            //是否包含“()”    
                            if (strleft.IndexOf("(") == 0 && strright.IndexOf(")") == 0)
                            {
                                //是否为文件序号
                                if (rightIndex - leftIndex == 2)
                                {
                                    int sratrSort = 0;
                                    //取出序号值
                                    string strSort = strFileName.Substring(leftIndex + 1, 1);
                                    int.TryParse(strSort, out sratrSort);
                                    //起始序号从1开始
                                    if (sratrSort > 1)
                                    {
                                        if (sratrSort >= fileSort)
                                        {
                                            fileSort = sratrSort + 1;
                                        }
                                    }
                                    else
                                    {
                                        fileSort = sratrSort + 1;
                                    }
                                }
                            }
                        }
                        _excelFile = strSubName + "(" + fileSort + ")" + strExtenName;
                        filePath = strFilePath + "\\" + _excelFile;
                    }
                }

                //创建工作簿
                HSSFWorkbook workbook = new HSSFWorkbook();
                ISheet sheet = workbook.CreateSheet(sheetName);
                int i = 0, j = 0, count = 0;
                #region 设置Excel表格标题
                IRow titleInfo = sheet.CreateRow(count);
                ICell cellTitle = titleInfo.CreateCell(0);
                cellTitle.SetCellValue(sheetName);
                ICellStyle titleStyle = workbook.CreateCellStyle();
                titleStyle.Alignment = HorizontalAlignment.Center;//水平对齐
                IFont titleFont = workbook.CreateFont();
                titleFont.FontHeightInPoints = 15;
                titleFont.Boldweight = short.MaxValue;//字体加粗
                titleStyle.SetFont(titleFont);
                cellTitle.CellStyle = titleStyle;
                #endregion

                count = count + 1;
                #region 表头
                IRow headRow = sheet.CreateRow(count);
                ICell cellHead = null;
                ICellStyle styleHead = workbook.CreateCellStyle();//创建样式对象 
                styleHead.Alignment = HorizontalAlignment.Center;//水平对齐
                styleHead.VerticalAlignment = VerticalAlignment.Center;//垂直对齐
                IFont font = workbook.CreateFont(); //创建一个字体样式对象 
                font.FontName = "宋体"; //和excel里面的字体对应 
                font.Color = new NPOI.HSSF.Util.HSSFColor.Red().Indexed;//颜色参考NPOI的颜色对照表(替换掉PINK()) 
                font.FontHeightInPoints = 10;//字体大小 
                font.Boldweight = short.MaxValue;//字体加粗 
                styleHead.SetFont(font); //将字体样式赋给样式对象
                sheet.SetColumnWidth(0, 20 * 256);//设置列宽
                for (j = 0; j < data.Columns.Count; ++j)
                {
                    cellHead = headRow.CreateCell(j);
                    cellHead.CellStyle = styleHead;
                    cellHead.SetCellValue(data.Columns[j].ColumnName);
                    sheet.SetColumnWidth(i, 17 * 256);
                }
                #endregion

                count = count + 1;
                #region 填充Excel内容
                for (i = 0; i < data.Rows.Count; ++i)
                {
                    IRow rowBody = sheet.CreateRow(count);
                    ICell CellBody = null;
                    ICellStyle bodyStyle = workbook.CreateCellStyle();//创建样式对象 
                    bodyStyle.Alignment = HorizontalAlignment.Center;//水平对齐
                    bodyStyle.VerticalAlignment = VerticalAlignment.Center;//垂直对齐
                    IFont fontBody = workbook.CreateFont(); //创建一个字体样式对象 
                    fontBody.FontName = "宋体"; //和excel里面的字体对应 
                    fontBody.Color = new NPOI.HSSF.Util.HSSFColor.Black().Indexed;//颜色参考NPOI的颜色对照表(替换掉PINK()) 
                    fontBody.FontHeightInPoints = 10;//字体大小 
                    fontBody.Boldweight = short.MinValue;//字体加粗
                    bodyStyle.SetFont(fontBody); //将字体样式赋给样式对象
                    for (j = 0; j < data.Columns.Count; ++j)
                    {
                        CellBody = rowBody.CreateCell(j);
                        CellBody.CellStyle = bodyStyle;
                        CellBody.SetCellValue(data.Rows[i][j].ToString());
                        sheet.SetColumnWidth(i, 17 * 256);
                    }
                    ++count;
                }
                #endregion

                //合并单元格
                sheet.AddMergedRegion(new NPOI.SS.Util.CellRangeAddress(0, 0, 0, data.Columns.Count));

                //生成文件 
                FileStream file = new FileStream(filePath, FileMode.Create);
                workbook.Write(file); file.Close();
                System.Web.HttpContext.Current.Response.AppendHeader("Content-Disposition", "attachment;filename=" + (!string.IsNullOrEmpty(_excelFile) ? _excelFile : "DownloadData." + strExtenName));
                System.Web.HttpContext.Current.Response.ContentType = "application/ms-excel";
                System.Web.HttpContext.Current.Response.WriteFile(filePath);
                System.Web.HttpContext.Current.Response.Flush();
                System.Web.HttpContext.Current.Response.End();
            }
            catch (Exception ex)
            {
                rMsg = "异常：" + ex.Message + " Detail:" + (ex.InnerException != null ? ex.InnerException.ToString() : "");
            }
            excelFile = _excelFile;
            return rMsg;
        }