How to use wp query in my custom page without including header and footer in wordpress

You have an option to use require() method in PHP. It doesn’t require to load header/footer in renderdata.php in order to use WP_Query() query class.

add_action( 'wp_ajax_nopriv_aj_page', 'my_aj_page' );
add_action( 'wp_ajax_aj_page', 'my_aj_page' );

function my_aj_page() {
    require('ajaxpage/renderdata.php');
    die();
}