<select id="a" onchange="show_input();">
<option value=''>--請選擇--</option>
<option value='1'>1</option>
<option value='2'>2</option>
<option value='3'>3</option>
</select>
<input type="text" id="b">

<script type="text/javascript">
function show_input(){
var select_value = document.getElementById('a').value;
document.getElementById('b').value = select_value;
}
</script>