示例：
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8" />
<title>test page</title>
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="css/main.css" />
<script src="./jquery.js"></script>
</head>
<body>
<div class="top"></div>
<div class="wrap">
<div class="tip">浮起来吧</div>
<div class="xia">mmmmmmmmmmmmmmmmmmbbbbbbbbbbbbbbbbbbbbbbb</div>
1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>
1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>
1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>
1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>1<br>
</div>
<script type="text/javascript">
$(function(){
var stiptop = $(".tip").position().top;
var sNum = 210;
$(window).scroll(function(){
var sTop = parseInt($(window).scrollTop());
if(sTop > 210){
$(".tip").css({'position':'fixed','marginTop':'0px'});
}
else{
$(".tip").css({'position':'static','marginTop':'10px'});
}
});
});
</script>
</body>
</html>