在主题的功能文件里functions.php添加以下代码，然后后台设置里的固定链接更新下就好了:

add_action('init', 'html_page_permalink', -1);
function html_page_permalink() {
    global $wp_rewrite;
    if ( !strpos($wp_rewrite->get_page_permastruct(), '.html')){
        $wp_rewrite->page_structure = $wp_rewrite->page_structure . '.html';
    }
}