Using rem.js is a cinch; you can load the polyfill conditionally using a loader like yepnope or simply by including a reference to it in your page like so: <script src="js/rem.js" type="text/javascript"></script>. As a best practice, you should either reference rem.js after all of your stylesheets or, better yet, at the end of your <body> tag.
我猜你说的rem.js应该是这个 REM unit polyfill
文档中有一句话
意思是你需要引入rem.js在页脚,也就是css文件,以及dom全部加载完再引入rem.js
ps:下次提问最好把代码,链接,图片一起发上来,还有chrome的调试信息,以及自己做过的尝试