做了一点小修改，希望对大家有用！！
<html>
<head>
<style>
<!--
.cls1 { position:absolute; left:250px; top:89px; width:216px; height:72px; z-index:1; }
.cls2 { position:absolute; left:250px; top:89px; width:95px; height:18px; z-index:2 }
input { font-size: 12px; padding-top: 2px; padding-left: 2px;width:127;}
//-->
</style>
</head>
<script>
<!--
function addOption(pos){
if(event.keyCode==13)
{
var select_obj = document.getElementById("myselect");
var text_value = document.getElementById("test").value;
if (select_obj.length)
{
for (var i=0;i<select_obj.length;i++)
{
// 如果已经存在，不添加，直接退出
if (select_obj.options[i].text==text_value)
alert("选项已存在，请重新输入");
}
}
var the_option= new Option(text_value,text_value);
select_obj.add(the_option);
select_obj.selectedIndex = select_obj.options.length-1
}
}
//-->
</script>
<div class="cls1" style="clip: rect(3 280 21 110)">
<select name="myselect" style="width:127" onchange="document.getElementById('test').value=this.value">
</select>
</div>
<div class="cls2">
<input type="text" name="test" onkeydown="addOption()" size="20" style="width: 127; height: 23">
</div>
</body>
</html>