.html代码如下：

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head>
    <title>下来列表级联效果编写</title>
    <style type="text/css">
        #reg{width:300px;height:50px;background:#fca;padding-top:5px;padding-left:10px}
    </style>
    <script type="text/javascript">
    function getDay(){
      var y = parseInt(document.getElementById("year").value);
      var m = parseInt(document.getElementById("month").value);
      var obj = document.getElementById("day");
      var rui = 0 ;
      var day = 0 ;
      
      if((y % 400 == 0 )||(y % 4 == 0 && y % 100 != 0 )){
         rui = 1;
      }
      switch(m){
         case 1:
         case 3:
         case 5:
         case 7:case 8:case 10:case 12:day = 3;
         break;
         case 4:case 6:case 9:case 11:day = 2;
         break;
         case 2: day = 0 ;
         break;
         
      }
      <span style="color:#ff0000;">while(obj.firstChild)
         obj.removeChild(obj.firstChild);//判断是否有一个孩子节点，有的话就清空。如果没这一句就会一直叠加</span>
      
      for(var i = 1 ; i <= 28 + rui + day ; i++){
         var option = document.createElement("option");
         option.innerText = i;
         obj.appendChild(option);
      }
      
    }
   
   function initYear(){
      /*var year = document.getElementById('year');
      var date = new Date();
      for(var i = 1900 ; i <= date.getFullYear(); i++){
         var option = document.createElement("option");
         option.innerText = i ; 
         year.appendChild(option);
      }*/
      
      
      
   }
    </script>
</head>
<body οnlοad="getDay()">
<form name="myForm">
<div id="reg" >
<table >
<tbody>
    
    <tr>
    <td>出生日期</td>
    <td><select id="year">
   <script type="text/javascript">
      var date = new Date();
      var str = "";
      for(var i = 1900 ; i <= date.getFullYear(); i++){
         str += "<option value='"+i+"' ";
         if( i == date.getFullYear()){
            str += " selected>"+i+"<option>";
         }else{
            str += ">"+i+"</option>";
         }
         document.write(str);
      }
   </script>
    </select>年<select οnclick="getDay()" id="month" >
   <script type="text/javascript">
   for(var i = 1 ; i <= 12 ; i++){
      document.write("<option value='"+i+"'>"+i+"</option>");
   }
   </script>
    </select >月<select id="day"></select>日</td>
    <td></td>
    </tr>
</tbody>
</table>
</div>
</form>
</body>
</html>


注意标红处。显示效果如下：

DOM实现动态添加级联菜单_javascript