<!DOCTYPE html>
<html>
<head>
<script type="text/javascript" src="http://code.jquery.com/jquery-latest.js"></script>
<style>
.aa
{
position: fixed;
left: 0px; //一定要
right: 0px; //一定要
200px; //一定要
height: 200px; //可选，会自动撑开
margin-left:auto; //一定要
margin-right:auto; //一定要
}
</style>
<script type="text/javascript">
</script>
</head>
<body>
<div class="aa" >
<div>很好看</div>
</div>
</body>
</html>