经过大大指点，用replace写确定简单了许多。

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>添加删除元素的className</title>
<style type="text/css">
    .box{ width:100px; height:100px; background:#C33;}
</style>

<script type="text/javascript">
window.onload=function(){
    var box=document.getElementById('box');
    addClassName(box,'aa');
    addClassName(box,'bb');
    addClassName(box,'cc');
    addClassName(box,'dd');
    
    delClassName(box,'cc')
    delClassName(box,'box')
    
};

function addClassName(obj,className){
    if(obj.className==''){
        obj.className=className;    
    }else{
        obj.className+=' '+className;    
    }
}
    
function delClassName(obj,className){
    className=className;
    var currentClassName=obj.className;
    if(currentClassName!=''){
        var nowClassName=currentClassName.replace(className,'');
        obj.className=nowClassName;
    }
}
</script>
</head>

<body>

<div class="box" id="box"></div>
</body>
</html>