drag.html

<div id="test" style="height: 100px;width: 100px;background:pink;position:absolute;top:0;left:0;"></div>
<script>
test.onmousedown = function(e){
    e = e || event;
    //获取元素距离定位父级的x轴及y轴距离
    var x0 = this.offsetLeft;
    var y0 = this.offsetTop;
    //获取此时鼠标距离视口左上角的x轴及y轴距离
    var x1 = e.clientX;
    var y1 = e.clientY;

    test.onmousemove = function(e){
        e = e || event;
        //获取此时鼠标距离视口左上角的x轴及y轴距离
        x2 = e.clientX;
        y2 = e.clientY;    
        //计算此时元素应该距离视口左上角的x轴及y轴距离
        var X = x0 + (x2 - x1);
        var Y = y0 + (y2 - y1);
        //将X和Y的值赋给left和top，使元素移动到相应位置
        test.style.left = X + 'px';
        test.style.top = Y + 'px';
    }

    test.onmouseup = function(e){
        //当鼠标抬起时，拖拽结束，则将onmousemove赋值为null即可
        test.onmousemove = null;
    }
}
</script>