(function(){			  var $backToTopTxt = "返回顶部", $backToTopEle = $('
    
').appendTo($("body"))		        .text($backToTopTxt).attr("title", $backToTopTxt).click(function() {		            $("html, body").animate({ scrollTop: 0 }, 120);		    	}), $backToTopFun = function() {		        var st = $(document).scrollTop(), winh = $(window).height();		        (st > 0)? $backToTopEle.show(): $backToTopEle.hide();		        //IE6下的定位		        if (!window.XMLHttpRequest) {		            $backToTopEle.css("top", st + winh - 166);		        }		    };		    $(window).bind("scroll", $backToTopFun);		    $(function() { $backToTopFun(); });		})();

CSS

.backToTop { display: none; width: 18px; line-height: 1.2; padding: 5px 0; background-color: #000; color: #fff; font-size: 12px; text-align: center; position: fixed; _position: absolute; right: 10px; bottom: 100px; _bottom: "auto"; cursor: pointer; opacity: .6; filter: Alpha(opacity=60);}

如果预览时左下角提示错误，请先刷新一下本页面就可以看到效果了。
  拖动滚动条到页面底部就会显示“返回顶部”按钮。
    

把JS代码 写在HTML下面