感谢@prosti的回答，我根据你的回答编写了这个小代码，它对我有用

global $post;
$query_args = array(
            \'post_type\' => \'post\',
            \'post_status\' => \'publish\',
            \'order\' => \'DESC\',
            \'posts_per_page\' => -1,
            \'cat\' => 1,
            // \'fields\' => \'SUM(amount_to_paid)\',
        );

$sum = 0;
$query = new WP_Query($query_args);
if ( $query->have_posts() ) {
    while( $query->have_posts() ) {
        $query->the_post();
        // do the processing for each post
        $sum = $sum + intval(get_post_meta(get_the_id(),\'download_counter\' ));
    }
}
echo $sum ;