H5页面用js控制缩放问题

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);
       if(version>2.3){
                document.write('<meta name="viewport" content="width=640, minimum-scale = '+phoneScale+', maximum-scale = '+phoneScale+', target-densitydpi=device-dpi">');
       }else{
                document.write('<meta name="viewport" content="width=640, target-densitydpi=device-dpi">');
       }
       } else {
               document.write('<meta name="viewport" content="width=640, user-scalable=no, target-densitydpi=device-dpi">');
      }

我看网上都有用这段js代码来控制页面百分百缩放，但是我在用的过程中发现页面切换不是百分百，就是说在切换的过程中，页面会先缩小再放大，求问大神们有解决的办法吗？