<p id="back-to-top"><a href="#top"><span></span></a></p>

p#back-to-top{
  position:fixed;
  bottom:100px;
  right:80px;
}
p#back-to-top a{
  text-align:center;
  text-decoration:none;
  color:#d1d1d1;
  display:block;
  width:30px;
  /*使用CSS3中的transition属性给跳转链接中的文字添加渐变效果*/
  -moz-transition:color1s;
  -webkit-transition:color1s;
  -o-transition:color1s;
}
p#back-to-top a:hover{
  color:#979797;
}
p#back-to-top a span{
  background:#d1d1d1 url(../images/arrow_up.png) no-repeat center center;
  border-radius:6px;
  display:block;
  height:30px;
  width:30px;
  margin-bottom:5px;
  /*使用CSS3中的transition属性给<span>标签背景颜色添加渐变效果*/
  -moz-transition:background1s;
  -webkit-transition:background1s;
  -o-transition:background1s;
}
#back-to-top a:hover span{
  background:#979797 url(../images/arrow_up.png) no-repeat center center;
}

<script type="text/javascript">
$(document).ready(function(){
 //首先将#back-to-top隐藏
 $("#back-to-top").hide();
 //当滚动条的位置处于距顶部600像素以下时，跳转链接出现，否则消失
 $(function () {
  $(window).scroll(function(){
   if ($(window).scrollTop()>600){
    $("#back-to-top").fadeIn(500);
   }else{
    $("#back-to-top").fadeOut(500);
   }
 });
 //当点击跳转链接后，回到页面顶部位置
 $("#back-to-top").click(function(){
  $('body,html').animate({scrollTop:0},500);
   return false;
  });
 });
});
</script>