WordPress and underconstruction page

Hello,
i have installed new wordpress to my site, and now i would like to display underconstruction page while i manage the content of my wp site.

the underconstruction page file name is: underconstruction.html
i put DirectoryIndex in .htaccess: directoryIndex underconstruction.html index.php

now the page display underconstruction page, but when i add index.php (www.domain.com/index.php)..it always display underconstruction..

i turn off permalink, rewrite .htaccess.. the problem still exist, is there any way out to display WP-site when there’s underconstruction page ?