hao can i routing subdomain?

Some things are simpler than they seem ...

Code:

// application/config/routes.php
switch ($_SERVER['HTTP_HOST'])
{
   case 'sub.domain.tld':
       // your sub.domain.tld routes here
       break;
   case 'default.domain.tld':
   default:
       // your default domain routes here
       break;
}

Wouldn't this only work if your subdomain is pointing to the same root directory as your main domain?

Yes, but how else do you expect to be able to have domain-based routing within CI?

I was asking because a subdomain by default points to it's own directory in the root of your hosting