<!DOCTYPE html>

<html lang="en">

<head>

    <meta charset="UTF-8">

    <title>Title</title>

    <script type="text/javascript">

        window.onload=function () //加载全部网页内容

        {

            var obtn = document.getElementById('btn'); //获取全选标签

            var ainput = document.getElementsByTagName('input');//获取全部input标签即获取全部复选框

            var i = 0;

            obtn.onclick=function ()//点击全部标签要执行下面事件

            {

                for (i=0;i<ainput.length;i++)

                {

                    ainput[i].checked=true;//重复选择复选框

                }

            }

        }

    </script>

</head>

<body>

    <p id="btn">全选</p>

    <input type="checkbox" />

    <input type="checkbox" />

</body>

</html>