<div id="box">
        <!-- toBig是放大鏡元素 -->
     <div id="toBig"></div>
        <!-- 小圖片 -->
  <img src="img/05.jpg" id="smallImg" width="800px">
</div>
<div id="beBig">
        <!-- 大圖片,比例為1.5倍數(shù) -->
  <img src="img/05.jpg" id="bigImg" width="1200px">
</div>

 *{
          margin: 0px;
          padding: 0px;
            }    
            #box{
             position: relative;
             float: left;
            }
   #toBig{
    width: 80px;
    height: 80px;
    border: 1px solid gray;
    background-color: transparent;
    position: absolute;
   }
   #beBig{
    float: left;
    overflow: hidden;
    border: 1px solid gray;
    position: relative;
    left: 40px;
    top:325px ;
   }
   #bigImg{
    position: absolute;
   }

<script type="text/javascript">
            //獲取小圖片,大圖片,放大鏡元素,大圖片的父元素
   var smallImg=document.querySelector("#smallImg");
   var bigImg=document.querySelector("#bigImg");
   var toBig=document.querySelector("#toBig");
   var beBig=document.querySelector("#beBig");
 
            /*在頁面加載時就先計算出小圖片與大圖片的比例*/
            var q=0;
   window.onload=function(){
    q=bigImg.offsetWidth/smallImg.offsetWidth;
                //根據(jù)放大鏡的寬高和比例計算要顯示放大內(nèi)容的大小
    beBig.style.width = toBig.clientWidth * q +"px";
    beBig.style.height = toBig.clientHeight * q +"px";
   }
            //獲取放大鏡元素的中心，保證鼠標在放大鏡中心
   var xCenter=toBig.clientWidth/2;
   var yCenter=toBig.clientHeight/2;
 
            //flag是一個標志,當鼠標按下時為true,可以進行移動
   flag=false;
   toBig.onmousedown = function(){
    flag=true;
   }
   toBig.onmouseup = function(){
    flag=false;
   }
   
   window.onmousemove=function(ev){
    var ev = ev || window.event;
                //flag為true時,放大鏡元素被按下并可以進行拖動
    if(flag){
                    //獲取鼠標當前所在位置并計算除了元素自身外要移動的位置
     var mouseX=ev.clientX,mouseY=ev.clientY;
     var trueX=mouseX - xCenter;
 
                    //判斷放大鏡元素是否超出小圖片范圍
     if(trueX < smallImg.offsetLeft){
      trueX = smallImg.offsetLeft;
     }
     if(trueX > smallImg.clientWidth - toBig.offsetWidth){
      trueX = smallImg.clientWidth - toBig.offsetWidth;
     }
     var trueY=mouseY - yCenter;
     if(trueY <= smallImg.offsetTop){
      trueY = smallImg.offsetTop;
     }
     if(trueY > smallImg.clientHeight - toBig.offsetHeight){
      trueY = smallImg.clientHeight - toBig.offsetHeight;
     }
 
                    //小圖片移動
     toBig.style.left = trueX + "px";
     toBig.style.top = trueY + "px";
     console.log(trueX,trueY);
     
     // 大圖片要移動的位置
     
     bigImg.style.left =-(trueX * q) + "px";
     bigImg.style.top =-(trueY * q) + "px";
    }
   }
   
</script>