How to go from Huanglong to Dujiangyan?

No direct transport is possible. I suggest you take a bus to Chuanzhusi first. Upon getting off, walk across the street to take a direct bus to Songpan or directly take a taxi there. I remember only one bus leaves Songpan Songzhou Bus Station to Dujiangyan and it departs at 07:20 in the morning. Othewise, you have to go back to Chengdu for a transfer.

Vice versa, Dujiangyan to Songpan, what bus terminal in Dujiangyan? Thank you.

One bus is scheduled for the return trip every morning at 07:00, and it leaves from Dujiangyan Passenger Transport Center.