1.需求:默认a2为选中!!

<select name="" id="chose" autocomplete="off">

    <option value="a0">a0</option>

    <option value="a1">a1</option>

    <option value="a2">a2</option>

    <option value="a3">a3</option>

</select>

document.getElementById('chose').getElementsByTagName('option')[2].selected = true;document.getElementById('chose')[2].selected = true;