<div id="fangdajing">
    <div class="small">
      <img src="small.jpg" alt="">
      <div class="shadow"></div>
    </div>
    <div class="big">
      <img src="big.jpg" alt="">
    </div>
  </div>

//定位，大图显示区域和阴影区域最开始不显示
#fangdajing{
      width:450px;
      height:450px;
      position:relative;
    }
    .small{
      width:450px;
      height:450px;
      position:absolute;
      left:0px;
      top:0px;
    }
    .shadow{
      width:200px;
      height:200px;
      background:yellow;
      opacity:0.3;
      position:absolute;
      top:0;
      left:0;
      display:none;
    }
    .big{
      position:absolute;
      left:450px;
      width:356px;
      height:356px;
      overflow:hidden;
      display:none;
    }

var fdj = document.getElementById('fangdajing');
var big = document.getElementsByClassName('big')[0];
var small = document.getElementsByClassName('small')[0];
var shadow = document.getElementsByClassName('shadow')[0];

  small.onmouseover = function(){
        big.style.display = 'block';
        shadow.style.display = 'block';
  }

   small.onmousemove = function(ent){
    var e = ent || event;  //获取鼠标事件对象
    var left = e.pageX - fdj.offsetLeft - shadow.offsetWidth/2; //获取shadow在small内的相对位置
    var top = e.pageY - fdj.offsetTop - shadow.offsetHeight/2;
    var tw = fdj.offsetWidth - shadow.offsetWidth; //获取shadow最大可移动距离
    var th = fdj.offsetHeight - shadow.offsetHeight; 
    //对shadow进行限制
    if(left < 0){
      left = 0;      
    }else if(left > tw){
      left = tw;
    }
     if(top < 0){
      top = 0;      
    }else if(top > th){
      top = th;
    } 
    //赋值
    small.style.left = left + 'px';
    small.style.top = top + 'px';    
    //大图跟着移动
    var sl = left * big.offsetWidth / shadow.offsetWidth;
    var st = top * big.offsetHeight / shadow.offsetHeight;
    big.scrollTop = st;
    big.scrollLeft = sl; 
  }

  small.onmouserout = function(){
    big.style.display = 'none';
    shadow.style.display = 'none';    
  }