rem 转 px

(function (doc, win) {
    var docEl = doc.documentElement,
        resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize',
        recalc = function () {
            var clientWidth = docEl.clientWidth;
            if (!clientWidth) return;
            if(clientWidth>=640){
                docEl.style.fontSize = '100px';
            }else{
                docEl.style.fontSize = 100 * (clientWidth / 640) + 'px';
            }
        };
    if (!doc.addEventListener) return;
    win.addEventListener(resizeEvt, recalc, false);
    doc.addEventListener('DOMContentLoaded', recalc, false);
})(document, window);

　　第二种方法

(function() {
    var newRem = function() {
        var html = document.documentElement;
        html.style.fontSize = html.getBoundingClientRect().width / 16 + 'px';
    };
    window.addEventListener('resize', newRem, false);
    newRem();
})();