Wordpress index.php redirect

I've just installed WordPress and I want to work on the theme. So I need to have an under construction page. I made an under.html file and addressed it in the .htaccess file as a default page. But when I want to go to mydomain.com/index.php to see the WordPress main page and work on the theme, It redirects to mydomain.com again and shows me that under.html. Any solution to have both under.html as the default page and the wordpress first page at the same time?