尝试使用此功能

function get_posts_in_taxonomy($post_type, $taxonomy, $term_id, $include_children=false, $limit)
{
$args = [
    \'post_type\'      => $post_type,
    \'posts_per_page\' => $limit,
    \'tax_query\' => [
        [
            \'taxonomy\'         => $taxonomy,
            \'terms\'            => $term_id,
            \'include_children\' => $include_children
        ],
    ],
    // Rest of your arguments
];

$posts = new WP_Query( $args );

if($posts->have_posts()) {
    while($posts->have_posts()) {
        $posts->the_post();

        //code todo here

    }
}
} //end func

function get_the_terms( $post, $taxonomy ) {
if ( ! $post = get_post( $post ) ) {
    return false;
}

$terms = get_object_term_cache( $post->ID, $taxonomy );
if ( false === $terms ) {
    $terms = wp_get_object_terms( $post->ID, $taxonomy );
    if ( ! is_wp_error( $terms ) ) {
        $term_ids = wp_list_pluck( $terms, \'term_id\' );
        wp_cache_add( $post->ID, $term_ids, $taxonomy . \'_relationships\' );
    }
}

/**
 * Filters the list of terms attached to the given post.
 *
 * @since 3.1.0
 *
 * @param WP_Term[]|WP_Error $terms    Array of attached terms, or WP_Error on failure.
 * @param int                $post_id  Post ID.
 * @param string             $taxonomy Name of the taxonomy.
 */
$terms = apply_filters( \'get_the_terms\', $terms, $post->ID, $taxonomy );

if ( empty( $terms ) ) {
    return false;
}

return $terms;}

我想我把自己弄糊涂了事实上，我是这样做的：

$_terms = wp_get_post_terms($post->ID, \'sub-category\', array("fields" => "all"));
        foreach ($_terms as $term) :
            $term_slug = $term->slug;
            $_posts = new WP_Query( array(
                        \'post_type\'         => \'post\',
                        \'posts_per_page\'    => 4,
                        \'tax_query\' => array(
                            array(
                                \'taxonomy\' => \'sub-category\',
                                \'field\'    => \'slug\',
                                \'terms\'    => $term_slug,
                            ),
                        ),
                    ));
            if( $_posts->have_posts() ) :
                while ( $_posts->have_posts() ) : $_posts->the_post();
                    //Output
                endwhile;
            endif;
            wp_reset_postdata();
            endforeach;