合并单元格的方法：
指定 4 个参数，起始行，结束行，起始列，结束列。然后这个区域将被合并。

CellRangeAddress region = new CellRangeAddress(startRow, endRow, startCol, endCol);
sheet.addMergedRegion(region);
合并的简单示例：

public class TestExcel {

public static void main(String[] args) throws IOException {

HSSFWorkbook workbook = new HSSFWorkbook();

HSSFCellStyle style = workbook.createCellStyle();
style.setAlignment(HSSFCellStyle.ALIGN_CENTER);
style.setVerticalAlignment(HSSFCellStyle.VERTICAL_CENTER);

HSSFSheet sheet = workbook.createSheet("sheet");

HSSFRow row0 = sheet.createRow(0);
HSSFCell cell_00 = row0.createCell(0);
cell_00.setCellStyle(style);
cell_00.setCellValue("日期");
HSSFCell cell_01 = row0.createCell(1);
cell_01.setCellStyle(style);
cell_01.setCellValue("午别");

HSSFRow row1 = sheet.createRow(1);
HSSFCell cell_10 = row1.createCell(0);
cell_10.setCellStyle(style);
cell_10.setCellValue("20180412");
HSSFCell cell_11 = row1.createCell(1);
cell_11.setCellStyle(style);
cell_11.setCellValue("上午");

HSSFRow row2 = sheet.createRow(2);
HSSFCell cell_21 = row2.createCell(1);
cell_21.setCellStyle(style);
cell_21.setCellValue("下午");

// 合并日期占两行(4个参数，分别为起始行，结束行，起始列，结束列)
// 行和列都是从0开始计数，且起始结束都会合并
// 这里是合并excel中日期的两行为一行
CellRangeAddress region = new CellRangeAddress(1, 2, 0, 0);
sheet.addMergedRegion(region);

File file = new File("E:\\demo.xls");
FileOutputStream fout = new FileOutputStream(file);
workbook.write(fout);
fout.close();
}

}

运行结果，得到的 Excel 表如下所示：

当然也可以更复杂些的，如下图，需要自己计算好行与列即可