<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title></title>
<style>
#goTop{
display:none;
position:fixed;
bottom:20px;
background:url(http://cdn.w3cfuns.com/resource/images/scrolltop.png) no-repeat 0 0;
width:34px;
height:34px;
cursor:pointer;
right:20px;
}
</style>
<script>
window.onload=function(){
var timer=null;
var goTop=document.getElementById("goTop");
goTop.onclick=function(){
timer=setInterval(function(){
var top=document.body.scrollTop+document.documentElement.scrollTop;
var speed=top/5;
if(document.body.scrollTop){
document.body.scrollTop-=speed;
}
else{
document.documentElement.scrollTop-=speed;
}
if(top==0){
clearInterval(timer);
}
},30);
};
window.onscroll=function(){
if(document.body.scrollTop+document.documentElement.scrollTop>=500){
goTop.style.display="block";
}
else{
goTop.style.display="none";
}
};
};
</script>
</head>
<body>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<p>123</p><br/>
<span id="goTop"></span>
</body>
</html>
原文：http://www.cnblogs.com/isylar/p/4843353.html