效果如下：
demo code:
<!DOCTYPE html>
<html>
<head lang="en">
<meta charset="UTF-8">
<title></title>
</head>
<body>
<select id="shengshi" onchange="asd(this)">
</select>
<select id="qu">
</select>
</body>
</html>
<script>
var arr=["北京","上海","深圳"];
var quArr=[];
quArr[0]=["丰台区","海淀区","朝阳区"];
quArr[1]=["上海1","上海2","上海3"];
quArr[2]=["深圳1","深圳2","深圳3"];
function asd(t){
qu.innerHTML="";
var diquArr=quArr[t.selectedIndex];
for(var i=0;i<diquArr.length;i++){
qu.add(new Option(diquArr[i],i),null);
}
}
window.onload=function(){
for(var i=0;i<arr.length;i++){
shengshi.add(new Option(arr[i],i),null);
}
}
</script>