确实有一个过滤器，walker_nav_menu_start_el 会帮你处理的！

以该代码为例：

function wpse356896_filter_primary_nav_menu_dropdown( $item_output, $item, $depth, $args ) {

    // Only for our primary menu location.
    if ( empty( $args->theme_location ) || \'primary-menu\' !== $args->theme_location ) {
        return $item_output;
    }

    // Add the dropdown for items that have children.
    if ( ! empty( $item->classes ) && in_array( \'menu-item-has-children\', $item->classes ) ) {
        return $item_output . \'<span class="dropdown"><img src="chevron.svg"></span>\';
    }

    return $item_output;
}

add_filter( \'walker_nav_menu_start_el\', \'wpse356896_filter_primary_nav_menu_dropdown\', 10, 4 );