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html代码

<div id="box">
    <div id="smallImg">
        <div id="shank"></div>
        <img id="small" src="img/1.jpg"/>
    </div>
    <div id="bigImg">
        <img id="big" src="img/1.jpg"/>
    </div>
</div>

css代码

*{
    margin: 0;
    padding: 0;
}
#box{
    width: 1000px;
    margin: 100px auto;
}
#smallImg{
    width: 300px;
    height: 300px;
    float: left;
    position: relative;
}
#small{
    width: 100%;
    height: 100%;
}
#shank{
    width: 150px;
    height: 150px;
    background: #000;
    opacity: 0.4;
    position: absolute;
    top: 0;
    left: 0;
    cursor: move;
    display: none;
}
#bigImg{
    width: 400px;
    height: 400px;
    overflow: hidden;
    margin-left: 20px;
    float: left;
    position: relative;
}
#big{
    width: 600px;
    height: 600px;
    display: none;
    position: absolute;
    top: 0;
    left: 0;
}

接下来js代码

var oSmallImg = document.getElementById("smallImg")
var oShank = document.getElementById("shank");
var oBig = document.getElementById("big");
var oBigImg = document.getElementById("bigImg");
oSmallImg.onmouseover = function(){
    oShank.style.display = 'block';
    oBig.style.display = 'block';
}
oSmallImg.onmouseout = function(){
    oShank.style.display = 'none';
    oBig.style.display = 'none';
}
oShank.onmousemove = function( ev ){
    ev = ev || event;
    var sL = ev.clientX - oSmallImg.offsetLeft - oShank.offsetWidth/2;
    var sT = ev.clientY - oSmallImg.offsetTop - oShank.offsetHeight/2;
    var w = oSmallImg.offsetWidth - oShank.offsetWidth;
    var h = oSmallImg.offsetHeight - oShank.offsetHeight;
    var bW = oBig.offsetWidth;
    var bH = oBig.offsetHeight;
    //设置百分比
    var preX = sL/w;
    var preY = sT/h;
    if( sL > w ){
        sL = w;
    }else if( sL < 0 ){
        sL = 0;
    };
    if( sT > h ){
        sT = h;
    }else if( sT < 0 ){
        sT = 0;
    };
    oShank.style.left = sL + 'px';
    oShank.style.top = sT + 'px';
    oBig.style.left = -preX*( bW - oBigImg.offsetWidth ) + 'px';
    oBig.style.top = -preY*( bH - oBigImg.offsetHeight ) + 'px';
};

效果图

放大镜.jpg