<script type="text/javascript">
document.getElementById("orange").selected=true;
<option id="apple">苹果</option>
<option id="orange">桔子</option>
<option id="pineapple" selected="selected">菠萝</option>
<option id="banana">香蕉</option>
<input type="button" onclick="selectOrange()" value="选择桔子">