1. <?php
2. $category = Mage::getModel('catalog/category');
3. if(is_object(Mage::registry('current_category'))){
4.     $current_category_path=Mage::registry('current_category')->getPathIds();
5. }else{
6.     $current_category_path = array();
7. }
8. $category->load(Mage::app()->getStore()->getRootCategoryId());
9. $children_string = $category->getChildren();
10. $children = explode(',',$children_string);
11. $extra_options='';
12. foreach($children as $c){
13.     $selected = (in_array($c, $current_category_path))?'SELECTED':'';
14.     $extra_options.= '<option value="' . $c . '" ' . $selected . '>' . $category->load($c)->getName() . '</option>' . "\n";
15. }
16. ?>
17. <form id="search_mini_form" action="<?php echo $this->helper('catalogSearch')->getResultUrl() ?>" method="get">
18.     <fieldset>
19.         <legend><?php echo $this->__('Search Site') ?></legend>
20.         <div class="mini-search">
21.             <input id="search" type="text" class="input-text" name="<?php echo $this->helper('catalogSearch')->getQueryParamName() ?>" value="<?php echo $this->helper('catalogSearch')->getEscapedQueryText() ?>" />
22.             <select name="cat" id="cat" class="input-text">
23.             <option value="">All Departments</option>
24.             <?= $extra_options ?>
25.            </select>
26.             <input type="submit" value="Go" style="border: 1px solid #808080;" alt="<?php echo $this->__('Search') ?>" />
27.             <div id="search_autocomplete" class="search-autocomplete"></div>
28.             <script type="text/javascript">
29.             //<![CDATA[
30.                 var searchForm = new Varien.searchForm('search_mini_form', 'search', '<?php echo $this->__('search site...') ?>');
31.                 searchForm.initAutocomplete('<?php echo $this->helper('catalogSearch')->getSuggestUrl() ?>', 'search_autocomplete');
32.             //]]>
33.             </script>
34.         </div>
35.     </fieldset>
36. </form>

在magento相应的模板中覆盖template/catalogsearch/form.mini.phtml文件即可~！！！！，然后刷新缓存，编译，你会发现搜索的旁边出来一个分类选项！