<div class="select_box" id="selected">
  <div class="select">
    <span>請選擇</span>
  </div>
  <ul class="list">
    <li>01</li>
    <li>02</li>
    <li>03</li>
    <li>04</li>
   </ul>
 </div>

CSS：

<style type="text/css">
   *{margin:0;padding:0}
   ul,ol{list-style: none}
   .select_box{
     position:relative;
     margin:100px auto;
     width:300px;
   }
   .select{
     padding:5px 10px;
     border:1px solid #dedede;
   }
   .select:hover{
     cursor:pointer;
   }
   .select span{
     display: block;
     background:url("../../img/downicon.png") no-repeat right;
   }
   .list{
     display: none;
     position:absolute;
     top:30px;
     width:298px;
     border:1px solid #dedede;
     border-top:none;
   }
   .list li{
     padding:5px 10px;
   }
   .list li:hover{
     background:#ddd;
   }
 </style>

JS：

 $(function(){
   $(".select").click(function(){
    $(".list").toggle();
   })
   $(".list li").click(function(){
    $(".select span").html($(this).html());
    $(".list").hide();
   })
   $(document).bind("click",function(e){
    var e = e || window.event;  //事件對象，兼容IE
    var target = e.target || e.srcElement; //源對象，兼容火狐和IE
    while(target){
      if (target.id && target.id == "selected"){  //循環(huán)判斷至根節(jié)點，防止點擊的是#selected和它的子元素
       return;
    }
      target = target.parentNode;
    }
      $(".list").hide();  //點擊的不是#selected和它的子元素，隱藏下拉菜單
    })
  })

效果：