首先感谢toscho 为他的解决方案。如果任何人都需要主题代码，而不是将其用作插件，那么这里将采用的代码重写为单个函数：

add_shortcode(\'wp_caption\', \'img_caption_add_description\');
add_shortcode(\'caption\', \'img_caption_add_description\');
function img_caption_add_description($attr, $content = null)
{
    $post_id = str_replace(\'attachment_\', \'\', $attr[\'id\']);
    $img = get_post((int)$post_id);

    if (is_a($img, \'WP_Post\')) {
        $attr[\'caption\'] = $img->post_content;
    }

    return img_caption_shortcode($attr, $content);
}

我找到了解决办法。我想在所有图片下显示描述。这个解决方案非常有效。幸亏https://wordpress.stackexchange.com/users/3687/tom-auger

<?php
/*
Plugin Name: WPSE-45326 Gallery Replacement example
Plugin URI: https://wordpress.stackexchange.com/questions/45326
Description: A plugin to demonstrate how to replace the default \'gallery\' shortcode and add additional HTML tags for more customization.
Version: 1.0
Author: Tom Auger
Author URI: http://www.tomauger.com
License: GPL2
*/

class wpse_45326_Gallery_Replacement {
    function __construct(){
        // Hook on the plugins-loaded action since it\'s the first real action hook that\'s available to us.
        // However, if you\'re using a theme and want to replace that theme\'s `gallery` custom shortcode,
        // you may need to use another action. Search through your parent theme\'s files for \'gallery\' and see
        // what hook it\'s using to define it\'s gallery shortcode, so you can make sure this code runs AFTER their code.
        add_action( "init", array( __CLASS__, "init" ) );
    }

    function init(){
        remove_shortcode( \'gallery\' ); // Remove the default gallery shortcode implementation
        add_shortcode( \'gallery\', array( __CLASS__, "gallery_shortcode" ) ); // And replace it with our own!
    }

    /**
    * The Gallery shortcode.
    *
    * This has been taken verbatim from wp-includes/media.php. There\'s a lot of good stuff in there.
    * All you want to do is add some more HTML to it, and since (for some reason) they didn\'t provide more
    * filters to be able to add, we have to replace the Gallery shortcode wholesale.
    *
    * @param array $attr Attributes of the shortcode.
    * @return string HTML content to display gallery.
    */
    function gallery_shortcode($attr) {
        global $post;

        static $instance = 0;
        $instance++;

        $output = apply_filters(\'post_gallery\', \'\', $attr);
        if ( $output != \'\' )
            return $output;

        if ( isset( $attr[\'orderby\'] ) ) {
            $attr[\'orderby\'] = sanitize_sql_orderby( $attr[\'orderby\'] );
            if ( !$attr[\'orderby\'] )
                unset( $attr[\'orderby\'] );
        }

        // NOTE: These are all the \'options\' you can pass in through the shortcode definition, eg: [gallery itemtag=\'p\']
        extract(shortcode_atts(array(
            \'order\'      => \'ASC\',
            \'orderby\'    => \'menu_order ID\',
            \'id\'         => $post->ID,
            \'itemtag\'    => \'dl\',
            \'icontag\'    => \'dt\',
            \'captiontag\' => \'dd\',
            \'columns\'    => 3,
            \'size\'       => \'thumbnail\',
            \'include\'    => \'\',
            \'exclude\'    => \'\',
            // Here\'s the new options stuff we added to the shortcode defaults
            \'titletag\'  => \'p\',
            \'descriptiontag\' => \'p\'
        ), $attr));

        $id = intval($id);
        if ( \'RAND\' == $order )
            $orderby = \'none\';

        if ( !empty($include) ) {
            $include = preg_replace( \'/[^0-9,]+/\', \'\', $include );
            $_attachments = get_posts( array(\'include\' => $include, \'post_status\' => \'inherit\', \'post_type\' => \'attachment\', \'post_mime_type\' => \'image\', \'order\' => $order, \'orderby\' => $orderby) );

            $attachments = array();
            foreach ( $_attachments as $key => $val ) {
                $attachments[$val->ID] = $_attachments[$key];
            }
        } elseif ( !empty($exclude) ) {
            $exclude = preg_replace( \'/[^0-9,]+/\', \'\', $exclude );
            $attachments = get_children( array(\'post_parent\' => $id, \'exclude\' => $exclude, \'post_status\' => \'inherit\', \'post_type\' => \'attachment\', \'post_mime_type\' => \'image\', \'order\' => $order, \'orderby\' => $orderby) );
        } else {
            $attachments = get_children( array(\'post_parent\' => $id, \'post_status\' => \'inherit\', \'post_type\' => \'attachment\', \'post_mime_type\' => \'image\', \'order\' => $order, \'orderby\' => $orderby) );
        }

        if ( empty($attachments) )
            return \'\';

        if ( is_feed() ) {
            $output = "\\n";
            foreach ( $attachments as $att_id => $attachment )
                $output .= wp_get_attachment_link($att_id, $size, true) . "\\n";
            return $output;
        }

        $itemtag = tag_escape($itemtag);
        $captiontag = tag_escape($captiontag);
        $columns = intval($columns);
        $itemwidth = $columns > 0 ? floor(100/$columns) : 100;
        $float = is_rtl() ? \'right\' : \'left\';

        $selector = "gallery-{$instance}";

        $gallery_style = $gallery_div = \'\';
        if ( apply_filters( \'use_default_gallery_style\', true ) )
            $gallery_style = "
            <style type=\'text/css\'>
                #{$selector} {
                    margin: auto;
                }
                #{$selector} .gallery-item {
                    float: {$float};
                    margin-top: 10px;
                    text-align: center;
                    width: {$itemwidth}%;
                }
                #{$selector} img {
                    border: 2px solid #cfcfcf;
                }
                #{$selector} .gallery-caption {
                    margin-left: 0;
                }
            </style>
            <!-- see gallery_shortcode() in wp-includes/media.php -->";
        $size_class = sanitize_html_class( $size );
        $gallery_div = "<div id=\'$selector\' class=\'gallery galleryid-{$id} gallery-columns-{$columns} gallery-size-{$size_class}\'>";
        $output = apply_filters( \'gallery_style\', $gallery_style . "\\n\\t\\t" . $gallery_div );

        $i = 0;
        foreach ( $attachments as $id => $attachment ) {
            $link = isset($attr[\'link\']) && \'file\' == $attr[\'link\'] ? wp_get_attachment_link($id, $size, false, false) : wp_get_attachment_link($id, $size, true, false);

            $output .= "<{$itemtag} class=\'gallery-item\'>";
            $output .= "
                <{$icontag} class=\'gallery-icon\'>
                    $link
                </{$icontag}>";

            // MODIFICATION: include the title and description HTML if we\'ve supplied the relevant shortcode parameters (titletag, descriptiontag)
            if ( $captiontag ) {
                $output .= "
                    <{$captiontag} class=\'wp-caption-text gallery-caption\'>";
                // The CAPTION, if there is one
                if ( trim( $attachment->post_excerpt ) ) {
                    $output .= "
                        " . wptexturize($attachment->post_excerpt);
                }

                // The TITLE, if we\'ve not made the \'titletag\' param blank
                if ( $titletag ){
                    $output .= "
                        <{$titletag} class=\\"gallery-item-title\\">" . $attachment->post_title . "</{$titletag}>";
                }

                // The DESCRIPTION, if we\'ve not specified a blank \'descriptiontag\'
                if ( $descriptiontag ){
                    $output .= "
                        <{$descriptiontag} class=\\"gallery-item-description\\">" . wptexturize( $attachment->post_content ) . "</{$descriptiontag}>";
                }

                $option .= "
                    </{$captiontag}>";
            }
            $output .= "</{$itemtag}>";
            if ( $columns > 0 && ++$i % $columns == 0 )
                $output .= \'<br style="clear: both" />\';
        }

        $output .= "
                <br style=\'clear: both;\' />
            </div>\\n";

        return $output;
    }
}

new wpse_45326_Gallery_Replacement();