Do you know if there is a night bus from Chengdu to Jiuzhaigou?

Oh yes. At the Chadianzi Bus Station, a bus which is scheduled at 19:10 can be your choice. It also is the last bus going to Jiuzhaigou. It shall take 8-9h to arrive and the fare is 140RMB.

Do you know if the bus ticket should be bought in advance?
I guess we can get it straight from the bus station, isn't it?

Well, the website doesn't have English version. So you don't need to book them in advance. Just purchase them at the bus station. Don't worry.

Many thanks. We will take the night bus you mentioned.
Is it possible to stop at Jiuzhaigou park entrance?
Otherwise I think it stops to far away (Jiuzhaigou bus station)

Hello Fabien. The 19:10 bus will terminate at Jiuzhaigou Bus Station in the county, but it will pass the valley on the way. Usually many passengers will get off at the valley stop which is close to the main entrance of Jiuzhaigou Valley. To ensure your smooth trip, it is best to inform the driver of your drop-off in advance.

By any chance do you know the bus station name or an hotel close to it?

Just tell the driver you want to get off at Jiuzhaigou Valley, not Jiuzhaigou Bus Station. There should be many other passangers getting off there. No worry.

Thanks ! Do you think drivers understand English or it's better to prepare some papers with Chinese written destinations?

I am afraid that the drivers may not understand English. It will be better to prepare some Chinese notes, like "您好，我想在九寨沟口下车，谢谢！"