第一种方式： ps 不用除以2 
     
 
        
        
        
        
        
        
        
        
        
        
      
     
 第二种方式：用rem   ps 除以2

// rem

(function(doc, win) {

      var docEle = doc.documentElement,

        evt = "onorientationchange" in window ? "orientationchange" : "resize",

        fn = function() {

        var width = docEle.clientWidth;

        width && (docEle.style.fontSize = 100 * (width / 720) + "px");

        }

      win.addEventListener(evt, fn , false);

      doc.addEventListener("DOMContentLoaded", fn, false)

    }(document, window))