通過修改select的value值

 1 <input type="button" id="btn" value="按鈕"/>
 2 <select name="select" id="select">
 3     <option value="1">aa</option>
 4     <option value="a">bb</option>
 5     <option value="c">cc</option>
 6 </select>
 7 <script type="text/javascript">
 8 //點擊按鈕，讓第二項選中
 9     document.getElementById('btn').onclick=function(){
10         document.getElementById('select').value="a";
11     }    
12 </script>