这将添加类sub-menu-0, sub-menu-1 等等，这取决于它们的深度。

//This will go in functions.php file
class wpse61220_submenu_class extends Walker_Nav_Menu {
  function start_lvl(&$output, $depth) {
    $indent = str_repeat("\\t", $depth);
    $output .= "\\n$indent<ul class=\\"sub-menu-".$depth."\\">\\n";
  }
}

//  Usage -
//  wp_nav_menu( array(\'walker\' => new wpse61220_submenu_class() ) );