偶然发现，不敢独享，与大家分享之

    <!--移动端版本兼容 -->
    <script type="text/javascript">
        var mengvalue = -1;
        //if(mengvalue<0){mengvalue=0;}
        var phoneWidth = parseInt(window.screen.width);
        var phoneScale = phoneWidth / 640;
        var ua = navigator.userAgent;
        if (/Android (\d+\.\d+)/.test(ua)) {
            var version = parseFloat(RegExp.$1);
            // andriod 2.3
            if (version > 2.3) {
                document.write('<meta name="viewport" content="width=640, minimum-scale = ' + phoneScale + ', maximum-scale = ' + phoneScale + ', target-densitydpi=device-dpi">');
                // andriod 2.3以上
            } else {
                document.write('<meta name="viewport" content="width=640, target-densitydpi=device-dpi">');
            }
            // 其他系统
        } else {
            document.write('<meta name="viewport" content="width=640, user-scalable=no, target-densitydpi=device-dpi">');
        }
    </script><meta name="viewport" content="width=640, user-scalable=no, target-densitydpi=device-dpi">
    <!--移动端版本兼容 end -->

原理很简单，根据设备尺寸动态在dom里设置viewport