你可以使用这个代码或者改编为适合你的工作案例，希望它能工作..把它添加到你的函数. php中

add_filter( 'woocommerce_product_related_posts', 'woocommerce_get_direct_related_products' );

function woocommerce_get_direct_related_products() {
    global $woocommerce, $product;

    // Related products are found from category
    $cats_array = array(0);

    // Get categories
    $terms = wp_get_post_terms( $product->id, 'product_cat' );

    //Select only the category which doesn't have any children
    foreach ( $terms as $term ) {
        $children = get_term_children( $term->term_id, 'product_cat' );
        if ( !sizeof( $children ) )
            $cats_array[] = $term->term_id;
    }

    // Don't bother if none are set
    if ( sizeof( $cats_array ) == 1 ) return array();

    // Meta query
    $meta_query = array();
    $meta_query[] = $woocommerce->query->visibility_meta_query();
    $meta_query[] = $woocommerce->query->stock_status_meta_query();

    $limit = 5;

    // Get the posts
    return array(
        'orderby'       => 'rand',
        'posts_per_page'=> $limit,
        'post_type'     => 'product',
        'fields'        => 'ids',
        'meta_query'    => $meta_query,
        'tax_query'     => array(
            array(
                'taxonomy'  => 'product_cat',
                'field'     => 'id',
                'terms'     => $cats_array
            )
        )
    );
}

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这可能是Woocommerce: Only show related products from same subcategory的重复项
另一种解决方法是覆盖模板文件[your-theme]/woocommerce/single-product/related.php
以下内容已更新，可与Woocommerce 3.x一起使用

<?php
/**
 * Related Products
 *
 * This template can be overridden by copying it to yourtheme/woocommerce/single-product/related.php.
 *
 * HOWEVER, on occasion WooCommerce will need to update template files and you
 * (the theme developer) will need to copy the new files to your theme to
 * maintain compatibility. We try to do this as little as possible, but it does
 * happen. When this occurs the version of the template file will be bumped and
 * the readme will list any important changes.
 *
 * @see         https://docs.woocommerce.com/document/template-structure/
 * @author      WooThemes
 * @package     WooCommerce/Templates
 * @version     3.9.0
 */

if ( ! defined( 'ABSPATH' ) ) {
  exit;
}

global $product, $woocommerce_loop;

if ( empty( $product ) || ! $product->exists() ) {
  return;
}

if ( ! $related = $product->get_related( $posts_per_page ) ) {
  return;
}

// Get ID of current product, to exclude it from the related products query
$current_product_id = $product->get_id();

$cats_array = array(0);

// get categories
$terms = wp_get_post_terms( $product->id, 'product_cat' );

// select only the category which doesn't have any children
foreach ( $terms as $term ) {
  $children = get_term_children( $term->term_id, 'product_cat' );
  if ( !sizeof( $children ) )
  $cats_array[] = $term->term_id;
}

$args = apply_filters( 'woocommerce_related_products_args', array(
  'post_type' => 'product',
  'post__not_in' => array( $current_product_id ),   // exclude current product
  'ignore_sticky_posts' => 1,
  'no_found_rows' => 1,
  'posts_per_page' => $posts_per_page,
  'orderby' => $orderby,
  'tax_query' => array(
    array(
        'taxonomy' => 'product_cat',
        'field' => 'id',
        'terms' => $cats_array
    ),
  )
));

$products                    = new WP_Query( $args );
$woocommerce_loop['name']    = 'related';
$woocommerce_loop['columns'] = apply_filters( 'woocommerce_related_products_columns', $columns );

if ( $products->have_posts() ) : ?>

  <section class="related products">

    <?php
    $heading = apply_filters( 'woocommerce_product_related_products_heading', __( 'Related products', 'woocommerce' ) );

    if ( $heading ) :
    ?>
      <h2><?php echo esc_html( $heading ); ?></h2>
    <?php endif; ?>

    <?php woocommerce_product_loop_start(); ?>

      <?php while ( $products->have_posts() ) : $products->the_post(); ?>

        <?php wc_get_template_part( 'content', 'product' ); ?>

      <?php endwhile; // end of the loop. ?>

    <?php woocommerce_product_loop_end(); ?>

  </section>

<?php endif;

wp_reset_postdata();

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我已经覆盖了子主题的相关产品文件，并添加了以下自定义代码
第一个相关的产品：yourtheme/woopherce/single-product/related.php
代码：

<?php
/**
 * Related Products
 *
 * This template can be overridden by copying it to yourtheme/woocommerce/single-product/related.php.
 *
 * HOWEVER, on occasion WooCommerce will need to update template files and you
 * (the theme developer) will need to copy the new files to your theme to
 * maintain compatibility. We try to do this as little as possible, but it does
 * happen. When this occurs the version of the template file will be bumped and
 * the readme will list any important changes.
 *
 * @see         https://docs.woocommerce.com/document/template-structure/
 * @package     WooCommerce\Templates
 * @version     3.9.0
 */

if ( ! defined( 'ABSPATH' ) ) {
    exit;
}

global $product, $woocommerce_loop;

if ( empty( $product ) || ! $product->exists() ) {
  return;
}

if ( ! $related = $product->get_related( $posts_per_page ) ) {
  return;
}

// Get ID of current product, to exclude it from the related products query
$current_product_id = $product->get_id();

$cats_array = array(0);

// get categories
$terms = wp_get_post_terms( $product->get_id(), 'product_cat' );

// select only the category which doesn't have any children
foreach ( $terms as $term ) {
  $children = get_term_children( $term->term_id, 'product_cat' );
  if ( !sizeof( $children ) )
  $cats_array[] = $term->term_id;
}

$args = apply_filters( 'woocommerce_related_products_args', array(
  'post_type' => 'product',
  'post__not_in' => array( $current_product_id ),   // exclude current product
  'ignore_sticky_posts' => 1,
  'no_found_rows' => 1,
  'posts_per_page' => 5,
  'orderby' => $orderby,
  'tax_query' => array(
    array(
        'taxonomy' => 'product_cat',
        'field' => 'id',
        'terms' => $cats_array
    ),
  )
));

$products                    = new WP_Query( $args );
$woocommerce_loop['name']    = 'related';
$woocommerce_loop['columns'] = apply_filters( 'woocommerce_related_products_columns', $columns );

if ( $products->have_posts() ) :

if ( $related_products ) : ?>

    <section class="related products">

        <?php
        $heading = apply_filters( 'woocommerce_product_related_products_heading', __( 'Related products', 'woocommerce' ) );

        if ( $heading ) :
            ?>
            <h2 class="hrtechpro-title"><?php echo esc_html( $heading ); ?></h2>
        <?php endif; ?>
        
        <?php woocommerce_product_loop_start(); ?>

            <?php while ( $products->have_posts() ) : $products->the_post(); ?>

                    <?php

                    wc_get_template_part( 'content', 'product' );
                    
                    ?>

            <?php endwhile; wp_reset_postdata();?>

        <?php woocommerce_product_loop_end(); ?>

    </section>
    <?php
endif;

endif;

wp_reset_postdata();

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如果你已经尝试了这段代码，并且工作成功，不要忘记支持这一努力。