好的，找到答案了！我补充道\'query_id\'我的查询中的参数

$query_string   = sanitize_text_field( wp_unslash( $_GET[\'searchString\'] ) );
$page_no        = intval( $_GET[\'pageNo\'] );
$posts_per_page = 9;
$offset         = ( $page_no - 1 ) * $posts_per_page;

$search_args = array(
    \'posts_per_page\' => $posts_per_page,
    \'post_status\'    => \'publish\',
    \'perm\'           => \'readable\',
    \'s\'              => $query_string,
    \'offset\'         => $offset,
    \'query_id\'       => \'my_custom_ajax_search\',
);

$search_results = new WP_Query( $search_args );

add_filter( \'posts_orderby\', \'my_order_search_by_posttype\', 10, 2 );

function my_order_search_by_posttype( $orderby, $wp_query ) {
    if ( ! is_admin() && ( is_search() || isset( $wp_query->query[\'query_id\'] ) && $wp_query->query[\'query_id\'] === \'my_custom_ajax_search\' ) ) {
...
    }
}