fontSize.js

var Global={
    init:function() {
        this.htmlFontSize();
    },
    //设置根元素的font-size
    htmlFontSize:function(){
        var doc = document;
        var win = window;
        function initFontSize(){
            var docEl = doc.documentElement,
                resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize',
                recalc = function(){
                    var clientWidth = docEl.clientWidth;
                    if(!clientWidth) return;
                    if(clientWidth>640){
                        clientWidth=640;
                    }
                    fontSizeRate = (clientWidth / 320);
                    var baseFontSize = 50*fontSizeRate;
                    docEl.style.fontSize =  baseFontSize + 'px';
                }
            recalc();
            if(!doc.addEventListener) return;
            win.addEventListener(resizeEvt, recalc, false);
            doc.addEventListener('DOMContentLoaded', recalc,false);
        }
        initFontSize();
    }

};
Global.init();

 如果psd的宽度是750px，那么640换成750，相应的320换成375。