<script>
function clearOption(obj,e){
var currKey=0,e=e||event;
currKey=e.keyCode||e.which||e.charCode;
if(currKey == 8){
obj.options[0].text = "";
}
}
function writeSelect(obj,e){
var currKey=0,e=e||event;
currKey=e.keyCode||e.which||e.charCode;
obj.options[0].selected = "select";
if(currKey != 8){//这个地方是为了避免在firefox又出现令人厌烦的乱码,可以试试注释这句话，看看在firefox的效果
obj.options[0].text = obj.options[0].text + String.fromCharCode(currKey);
}
e.returnValue=false;
return obj.options[0].text;
}
function ttt(){
var jg = document.getElementById("aa").options[document.getElementById("aa").selectedIndex].text;
//当然也可以在前面的writeSelect函数里面同时设置options的value，这样的话，直接可以在该程序里面用value取出
jQuery("#aa").empty();
jQuery("#aa").append("<option value=''></option>");
}

</script>

<select style='width:150px;z-index:-1' id="aa" name="selectHelpCode" onkeydown="clearOption(this,event)" onkeypress="writeSelect(this,event)">
<option value=""></option>
<option value="11">11</option>
<option value="22">22</option>
<option value="33">33</option>
</select>
<input type="button" value="清除" onclick="ttt();"/>