how to return from Dujiangyan Irrigation Project to Chengdu?

You may take a bus to Chadianzi Bus Station from the Passenger Transport Center. The buses are available from 6:00 to 19:00, and the fare is about CNY 16.

I am afraid that there is no direct bus to Xinnanmen Bus Station. But you can take bus line 82 from Chadianzi Bus Station to Binjiang Road Station. After that, walk east along Middle Binjiang Road for about 300 yards to Xinnanmen Bus Station. :)