Install codeigniter in parallels plesk server
I have figure out how to fix it at last.
First , make sure the controller class file name is in Capital letter. Lower case file name works in Apache but not the fastcgi.
Second, change the htaccess to :
RewriteEngine OnRewriteCond %{REQUEST_FILENAME} !-fRewriteCond %{REQUEST_FILENAME} !-dRewriteRule ^(.*)$ index.php/$1 [L]