//模态框垂直居中
 function centerModals() {
  $('.modal').each(
    function(i) {
     var $clone = $(this).clone().css('display', 'block')
       .appendTo('body');
     var top = Math.round(($clone.height() - $clone.find(
       '.modal-content').height()) / 2);
     top = top > 0 ? top : 0;
     $clone.remove();
     $(this).find('.modal-content').css("margin-top",
       top - 50);
    });
 }
 $('.modal').on('show.bs.modal', centerModals);
 $(window).on('resize', centerModals);

<!--登陆弹窗-->
 <form action="__URL__/doLogin" method="post" class="form-horizontal">

  <!--模态框声名-->
  <div class="modal" id="myModal" tabindex="-1">
   <!--窗口声名-->
   <div class="modal-dialog modal-sm">
    <!--内容声名-->
    <div class="modal-content">
     <div class="modal-header">
      <button class="close" data-dismiss="modal">
       <span>×</span>
      </button>
      <h4 class="modal-title">用户登录</h4>
     </div>

     <div class="modal-body">
      <div class="form-group has-success has-feedback">

       <label for="username" class="col-sm-3 control-label">账号</label>
       <div class="col-sm-9">
        <span class="glyphicon glyphicon-user form-control-feedback"
         aria-hidden="true"></span> <span id="inputSuccess3Status"
         class="sr-only">(success)</span> <input type="text"
         id="username" name="username" class="form-control"
         placeholder="请输入您的用户名" title="可包含中文数字和常用字符"
         onkeydown="if(event.keyCode==32||event.keyCode==13){return false;}">
        <!-- 禁用空格和回车 -->

       </div>
      </div>


      <div class="form-group has-success has-feedback">
       <label for="password" class="col-sm-3 control-label">密码</label>
       <div class="col-sm-9">
        <span class="glyphicon glyphicon-lock form-control-feedback"
         aria-hidden="true"></span> <span id="inputSuccess3Status"
         class="sr-only">(success)</span> <input type="password"
         id="password" name="password" class="form-control"
         maxlength="12" placeholder="请输入您的密码" oncopy="return false"
         oncut="return false" onpaste="return false" title=""
         onkeydown="if(event.keyCode==32||event.keyCode==13){return false;}">

       </div>
      </div>
     </div>
     <div class="modal-footer">
      <div class="form-group">
       <div class="col-sm-offset-2 col-sm-10">
        <button type="submit" class="btn btn-success" id="login">现在登录
         »</button>
       </div>
      </div>
     </div>
    </div>
   </div>
  </div>
 </form>