<!DOCTYPE html>
<html>
<head>
<style>
.addcss{
100px;
height: 50px;
}
</style>
<meta charset="UTF-8">
<title></title>
</head>
<body>
<select class="addcss" id="aaa" onchange="show()">
<option value="0">0</option>
<option value="1">1</option>
<option value="2">2</option>
</select>
此处style="display:none"为隐藏，block为显示
<select class="addcss" style="display:none" id="_select">
<option value="a">a</option>
<option value="b">b</option>
</select>
</body>
</html>
<script type="text/javascript">
function show(){
var ss=document.getElementById("aaa").value;
if (ss=="1") {
document.getElementById('_select').style.display='block';
} else{
document.getElementById('_select').style.display='none';
}
}
</script>