<html>
<head>
<script type="text/javascript">
function change(){
var a=document.getElementById(‘aa‘).value;
if(a==1 || a==2)
{
document.getElementById(‘bb‘).style="display:none;"
}
if(a==3 || a==4)
{
document.getElementById(‘bb‘).style="display:block;"
}
}
</script>
</head>
<body>
<select id="aa" onchange="change()">
<option value="1">aa</option>
<option value="2">bb</option>
<option value="3">cc</option>
<option value="4">dd</option>
</select>
</br>
<select id="bb" style="display:none;">
<option value="1">11</option>
<option value="2">22</option>
<option value="3">33</option>
<option value="4">44</option>
</select>
</body>
</html>
原文：http://www.cnblogs.com/qiuyang1/p/3979025.html