可以啊，你在對應的li上類名active即可。

<div?class="dropdown">
<button?type="button"?class="btn?dropdown-toggle"?id="dropdownMenu1"?
data-toggle="dropdown">
主題
<span?class="caret"></span>
</button>
<ul?class="dropdown-menu"?role="menu"?aria-labelledby="dropdownMenu1">
<li?class="active"??role="presentation">
<a?role="menuitem"?tabindex="-1"?href="#">Java</a>
</li>
<li?role="presentation">
<a?role="menuitem"?tabindex="-1"?href="#">數據挖掘</a>
</li>
<li?role="presentation">
<a?role="menuitem"?tabindex="-1"?href="#">數據通信/網絡</a>
</li>
<li?role="presentation"?class="divider"></li>
<li?role="presentation">
<a?role="menuitem"?tabindex="-1"?href="#">分離的鏈接</a>
</li>
</ul>
</div>

JavaScript設置value直接默認等于不就行了