<select id="dlt">
<option value="1" selected="selected">1</option>
<option value="2">2</option>
<option value="3">3</option>
</select>
<script type="text/javascript">
<!--
//方法1
dlt.value=2;
//方法2
dlt.selectedIndex = 2;
//方法3
var opt = dlt.options[0];
opt.selected = true;
//-->
</script>
<option value="1" selected="selected">1</option>
<option value="2">2</option>
<option value="3">3</option>
</select>
<script type="text/javascript">
<!--
//方法1
dlt.value=2;
//方法2
dlt.selectedIndex = 2;
//方法3
var opt = dlt.options[0];
opt.selected = true;
//-->
</script>
document.getElementById("下拉框的id").value = "想要选中的value";