<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>title</title>
    <style>
        *{
            margin: 0;
            padding: 0;
        }
        #box{
            background: black;
            width: 100px;
            height: 100px;
            border-radius: 10px;
            cursor: pointer;
            position: relative;
        }
    </style>
</head>
<body>
<div id="box"></div>

<script>
    window.onload = function () {
        var box = document.getElementById('box');
        var X = 0;
        var Y = 0;
        // 鼠标按下
        box.onmousedown = function (ev) {
            var oEvent = ev || event;
            // 鼠标在BOX中的横向距离 = 鼠标位置 - div位置
            X = oEvent.clientX - box.offsetLeft;
            // 鼠标在BOX中的纵向距离 = 鼠标位置 - div位置
            Y = oEvent.clientY - box.offsetTop;
            // 鼠标移动
            document.onmousemove = function (ev) {
                var oEvent = ev || event;
                box.style.left = oEvent.clientX - X + 'px';
                box.style.top = oEvent.clientY - Y + 'px';
            };
            // 鼠标抬起，清除所有事件，否则还会移动
            document.onmouseup = function () {
                document.onmousemove = null;
                document.onmouseup = null;
            };
        };
    };
</script>
</body>
</html>