<select onchange = "op(this)">
    <option value="" selected="selected"></option>
    <option value="1">第一组</option>
</select>
<select id="sel">
</select>

<script type="text/javascript">
    var a = ["aa","bb","cc"];
    var op = function(h){
        var sub = document.getElementById("sel");
        var set = function(){
            var frage = document.createDocumentFragment();
            for(var i=0,len=a.length;i<len;i++){
                var o = document.createElement("option");
                o.value = a[i];
                o.text = a[i];
                frage.appendChild(o);
            }
            sub.appendChild(frage);
        };
        sub.innerHTML = "";
        if(h.value == "1") set()
    };
</script>