var obj = document.getElementsByTagName('input');
 for (var i = 0; i < obj.length; i ++) {
     if (obj[i].name == 'id[1000]') 
         return obj[i];
 }

------解决方案--------------------
JScript code
<input type="text" name="div">
<form action="#" name="hell">
    <input type="text" name="id[1000]" />
</form>
<script type="text/javascript">
    window.onload=function(){
        var s=tab(document.getElementsByName("id[1000]")[0],"id[1000]");
        alert(s);
    }
    function tab(obj,str){
        var s=obj.parentNode;
        var tag=s.tagName.toLowerCase();
        if(tag!="body"){
            if(tag.name!=undefined)
                return tab(s,tag.name+"_"+str);
            else
                return tab(s,tag+"_"+str);
        }else{
            return "document_"+str;
        }
    }
</script>