默认情况下，对类别页面的查询将仅查询post post类型，因此除了在taxonomies参数中使用category注册post类型外，还必须通过pre_get_posts 要使CPT显示在类别页面上，请执行以下操作：

function wpa_illustrations_on_category_pages( $query ) {
    if ( $query->is_category() && $query->is_main_query() ) {
        $query->set( \'post_type\', array( \'post\', \'illustrations\' ) );
    }
}
add_action( \'pre_get_posts\', \'wpa_illustrations_on_category_pages\' );

Maybe try this:

    /** Add Custom Taxonomy **/
    function illustrations_category_taxonomy() {
        $labels = array(
            \'name\'              => __( \'Categories\' ),
            \'singular_name\'     => __( \'Category\' ),
            \'search_items\'      => __( \'Search Categories\' ),
            \'all_items\'         => __( \'All Categories\' ),
            \'parent_item\'       => __( \'Parent Category\' ),
            \'parent_item_colon\' => __( \'Parent Category:\' ),
            \'edit_item\'         => __( \'Edit Category\' ), 
            \'update_item\'       => __( \'Update Category\' ),
            \'add_new_item\'      => __( \'Add New Category\' ),
            \'new_item_name\'     => __( \'New Category\' ),
            \'menu_name\'         => __( \'Categories\' ),
        );
        $args = array(
            \'labels\'            => $labels,
            \'hierarchical\'      => true
        );
        register_taxonomy( \'illustrations-category\', \'illustrations\', $args );
    }
    add_action( \'init\', \'illustrations_category_taxonomy\');

您需要使用taxonomies argument 像这样：

    \'taxonomies\' => array(\'category\')

其他信息：http://codex.wordpress.org/Function_Reference/register_post_type#Arguments

<小时>

Update:

/* Register the \'Illustrations\' post type */

    add_action( \'init\', \'wpse114838_reg_pt_illustrations\');
    function wpse114838_reg_pt_illustrations() { 
        register_post_type(
            \'illustrations\',
            array(
                \'labels\' => array (
                \'name\' => \'Illustrations\',
                \'singular_name\' => \'Illustration\',
                \'menu_name\' => \'Illustrations\',
                \'add_new\' => \'Add Illustration\',
                \'add_new_item\' => \'Add new Illustration\',
                \'edit\' => \'Edit\',
                \'edit_item\' => \'Edit illustration\',
                \'new_item\' => \'New illustration\',
                \'view\' => \'View illustration\',
                \'view_item\' => \'View illustration\',
                \'search_items\' => \'Search illustrations\',
                \'not_found\' => \'No Illustrations Found\',
                \'not_found_in_trash\' => \'No Illustrations Found in Trash\',
                \'parent\' => \'Parent Illustration\',
                ),
                \'label\' => \'illustrations\',
                \'description\' => \'\',
                \'public\' => true,
                \'menu_icon\' => get_bloginfo( \'template_directory\' ) . \'/images/help-icon.png\',
                \'show_ui\' => true,
                \'show_in_menu\' => true,
                \'capability_type\' => \'post\',
                //you need just one of the methods, uncomment the one you want
                //\'taxonomies\' => array( \'category\' ),
                \'has_archive\' => true,
                \'hierarchical\' => true,
                \'rewrite\' => array( \'slug\' => \'\' ),
                \'query_var\' => true,
                \'supports\' => array( \'title\', \'editor\', \'custom-fields\', \'thumbnail\', \'page-attributes\', \'revisions\', \'comments\',\'author\' ),
            )
        );
        //you need just one of the methods, uncomment the one you want    
        //register_taxonomy_for_object_type( \'category\', \'illustrations\' );
    }