帮你改了一下  原因我不清楚 不过改完还是出错我也不太会 不过效果出来了

function changeTab(obj){

            var tabs = document.getElementsByClassName("tab");

            var tabs_active = document.getElementsByClassName("tab_active")[0];

            var contents = document.getElementsByClassName("content");

            var contents_active = document.getElementsByClassName("content_active")[0];

for(var i = 0; i<tabs.length; i++){

if(tabs[i] == obj){

var index = i;

tabs_active.className = "tab";

tabs[i].className = "tab tab_active";

contents[index].className = "content content_active";

               contents_active.className = "content";

}

}

}