<style type="text/css">
    #gotop{width:0.9rem;height:0.9rem;background:url(../images/gotop.png) no-repeat center top;background-size:0.9rem auto;position: fixed;bottom:20%;right:3%;cursor: pointer;z-index: 10;}
    #gotop span{
     background: gray;font-size: 35px;
    }

 </style>

 <!--侧边栏按钮-->
  <div class="mreturn" id="gotop">
    <a href="javascript:gotoTop()">
        <span class="layui-icon layui-icon-up"></span>
    </a>
</div>

function gotoTop(acceleration,stime) {
   acceleration = acceleration || 0.1;
   stime = stime || 10;
   var x1 = 0;
   var y1 = 0;
   var x2 = 0;
   var y2 = 0;
   var x3 = 0;
   var y3 = 0;
   if (document.documentElement) {
       x1 = document.documentElement.scrollLeft || 0;
       y1 = document.documentElement.scrollTop || 0;
   }

   if (document.body) {
       x2 = document.body.scrollLeft || 0;
       y2 = document.body.scrollTop || 0;
   }

   var x3 = window.scrollX || 0;
   var y3 = window.scrollY || 0; 

   // 滚动条到页面顶部的水平距离
   var x = Math.max(x1, Math.max(x2, x3));

   // 滚动条到页面顶部的垂直距离
   var y = Math.max(y1, Math.max(y2, y3)); 

   // 滚动距离 = 目前距离 / 速度, 因为距离原来越小, 速度是大于 1 的数, 所以滚动距离会越来越小
   var speeding = 1 + acceleration;
   window.scrollTo(Math.floor(x / speeding), Math.floor(y / speeding)); 

   // 如果距离不为零, 继续调用函数
   if(x > 0 || y > 0) {
       var run = "gotoTop(" + acceleration + ", " + stime + ")";
       window.setTimeout(run, stime);
   }

}