先把下面的代码拷贝到个html中，运行看效果
<html>
<head>
<script type="text/javascript">
var currentPosition,timer;
function GoTop(){
timer=setInterval("runToTop()",1);
}
function runToTop(){
currentPosition=document.documentElement.scrollTop || document.body.scrollTop;
currentPosition-=10;
if(currentPosition>0)
{
window.scrollTo(0,currentPosition);
}
else
{
window.scrollTo(0,0);
clearInterval(timer);
}
}
</script>
<style type="text/css">
</style>
</head>
<body>
<div style="height:1000px;text-align:center;font-size:200px;font-weight:bold;">饭</div>
<div style="height:1000px;text-align:center;font-size:200px;font-weight:bold;">吃</div>
<div style="height:1000px;text-align:center;font-size:200px;font-weight:bold;">家</div>
<div style="height:1000px;text-align:center;font-size:200px;font-weight:bold;">回</div>
<div style="height:1000px;text-align:center;font-size:200px;font-weight:bold;">你</div>
<div style="height:1000px;text-align:center;font-size:200px;font-weight:bold;">带</div>
<div id="back-up" onclick="GoTop()" style="border:1px solid red;height:100px;width:15px;position:fixed;cursor:pointer;right:10px;bottom:30px;">返回顶部</div>
<script>
window.scrollTo(0,document.body.scrollHeight);
</script>
</body>
</html>
速度可以自己控制哦。