方法1：
<head>
<style type="text/css">
#form_top{display:block; bottom:3px; right:3px; position:fixed;}
.top_img{
background-image: url("img/top.png");
width: 40px;
height:40px;
cursor: pointer;
}
</style>
</head>
<body>
<a id="_top"></a>
<div id="form_top"><a href="#_top" title="回到顶部"><div class="top_img">回到顶部</div></a></div>
</body>
方法2：
<body>
<div id="scrollA" class="intr_tit"></div>
<button id="ScrollToTop" class="btnimg Button2 WhiteButton Offscreen" type="button">返回<br>顶部</button>
<script type="text/javascript">
$(document).ready(function(){
ScrollToTop.setup();
});
ScrollToTop.setup=function(){
//var a=$(window).height()/5;
var a=$("#scrollA").offset().top;
//alert(a.top);
$(window).scroll(function(){
(window.innerWidth?window.pageYOffset:document.documentElement.scrollTop)>=a?$("#ScrollToTop").removeClass("Offscreen"):$("#ScrollToTop").addClass("Offscreen");
});
$("#ScrollToTop").click(function(){
$("html, body").animate({scrollTop:"0px"},400);
return false;
});
};
</script>
</body>