public function cate_display()
  {
    $cate = D('Cate'); 
    $field = array('id','cate_name','pid','sorts');
    $list = $cate->allCategory($field);
    $this->assign('list',$list);
    $this->display();
  }

public function allCategory($field='*'){
     $data = $this->field($field)->select();
     return $this->tree($data);
   }

public function tree($data,$pid=0,$level=0){
    static $tree = array();
    foreach($data as $k=>$v){
      if($v['pid'] == $pid){
        $v['level'] = $level;
        $tree[]=$v;
        $this->tree($data,$v['id'],$level+1);
      }
    }
  
    return $tree;
  }

<div class="form-group">
        <label for="pid" class="col-sm-2 control-label no-padding-right">上级菜单</label>
        <div class="col-sm-6">
          <select name="pid" style="width: 100%;">
            <option selected="selected" value="0">顶级菜单</option>
            <volist name="row" id="val"> 
              <option value="{$val.id}"><?php echo str_repeat('-',$val['level']*4); ?>{$val.cate_name}
              </option> 
            </volist> 
          </select>
        </div>
      </div>