1 #toTop {
 2        /*选中的背景图片的大小*/
 3     width: 54px;
 4     height: 54px;
 5     display: none;/*刚开始不显示*/
 6     position: fixed;
 7     right: 25px;/*距离右边大小值*/
 8     bottom: 45px;/*距离底部大小值*/
 9        /*选定图片区域*/
10     background-image: url(images/qqbg_1.5.5.png);
11     background-repeat: no-repeat;
12     background-position: -700px -110px;
13     opacity: 0.3; /*透明度*/
14 }
15 
16 #toTop:hover {
17     opacity: 1;/*悬停时不透明*/
18     filter: alpha(opacity = 100);
19 }

1 <a href=http://www.mamicode.com/"#" target="_self" id="toTop" title="返回顶部" onclick="window.scrollTo(0,0);return false"></a>
2 <script type = "text/javascript">
3     function toTopHide(){
4         $(document).scrollTop()>400?
5                 $("#toTop").show()
6                     :$("#toTop").hide();
7     }
8     $(window).scroll(function(){toTopHide()});
9 </script>