.box {
 width: 450px;
 height: 450px;
 margin: 100px 0 0 100px;
 border: 1px solid red;
 position: relative;
 }

 /* 右邊大盒子 */
 .bigBox{
 width: 540px;
 height: 540px;
 position: absolute;
 top: 100px;
 left: 560px;
 border: 1px solid #ccc;
 overflow: hidden;
 display: none;
 }
 .bigBox img {
 position: absolute;
 left: 0;
 top: 0;
 }


 /* 覆蓋物 */
 .box .mask{
 width: 260px;
 height: 260px;
 background-color: yellow;
 /*調(diào)整透明度 */
 opacity: .4;
 position: absolute;
 left: 0;
 top: 0;
 /* 默認(rèn)消失 */
 display: none;
 }

<script>
 
 window.onload = function (){
 var box = document.querySelector(".box");
 var mask = document.querySelector(".mask");
 var bigBox = document.querySelector(".bigBox");
 var bigImg = document.querySelector(".bigBox > img");
 console.log(bigImg);
 
 // 鼠標(biāo)移入
 box.onmouseover = function (){
 document.querySelector(".mask").style.display = "block";
 document.querySelector(".bigBox").style.display = "block";
 }
 // 移出
 box.onmouseout = function (){
 document.querySelector(".mask").style.display = "none";
 document.querySelector(".bigBox").style.display = "none";
 }

 // 讓覆蓋物跟隨鼠標(biāo)移動(dòng)
 box.onmousemove = function (){
 // 因?yàn)閎ox有100像素的外邊距，我們需要減掉，否則會(huì)錯(cuò)位
 var left = event.pageX - 100 - 130;
 var top = event.pageY - 100 - 130;
 // 覆蓋物不能超出box范圍，就需要判坐標(biāo)的取值范圍了
 // 得到覆蓋物在box中可移動(dòng)的最大距離
 var maskMax = this.offsetWidth - mask.offsetWidth;
 // 判斷l(xiāng)eft
 if(left <= 0){
 left = 0;
 }else if(left >= maskMax){
 left = maskMax;
 }

 // top
 if(top <= 0){
 top = 0;
 }else if(top >= maskMax){
 top = maskMax;
 }

 // 將設(shè)置好的left，top值設(shè)置給樣式
 mask.style.left = left + "px";
 mask.style.top = top + "px";
 mask.style.cursor = "move";

// 大圖在大盒子中的最大位移距離
var bigImgMax = bigImg.offsetWidth - bigBox.offsetWidth;
// 大圖片的left值
var bigImgX = mask.offsetLeft * bigImgMax / maskMax;
// 大圖的top值
 
var bigImgY = mask.offsetTop * bigImgMax / maskMax;
 // 覆蓋物與大圖片的移動(dòng)方向是反的，所以這里要給負(fù)值
 bigImg.style.left = -bigImgX + "px";
 bigImg.style.top = -bigImgY + "px";

 }
 }

<div class="box">
 <img src="../img/small.jpg" alt="">
<div class="mask">
</div>
<div class="bigBox">
 <img src="../img/big.jpg" alt="">
</div>