Two tickets, both needed
Entry to Jiuzhaigou National Park and the internal shuttle bus are sold as two items, and the bus is effectively compulsory: the valley is 30 to 40 km long in a Y shape, and walking the road between the valleys is not permitted. Buy both together.
Tickets are sold per day, not as a multi-day pass. If you want two days in the park you buy twice, and the second day is usually cheaper only in the sense that you already know where to go.
Booking
Book online in advance with real-name registration — passport number for foreign visitors — through the official Jiuzhaigou channels or the park's WeChat mini-program. There is a daily visitor cap, and in October it is reached every day. Tickets typically go on sale a few days to a week ahead, so set a reminder rather than assuming you can buy at the gate.
Bring the passport you booked with. It is checked at the entrance, and a booking under a different name will not get you in. Do not buy from touts at the car park; entry is by ID, so a resold ticket is worthless.
How the shuttle system works
Buses run on a hop-on, hop-off basis along the two upper valleys, Rize and Zechawa, which meet at Nuorilang. The standard strategy is to ride to the top of one valley at opening time, then work down on foot and by bus, transferring at Nuorilang to the other valley in the afternoon.
Which valley first is the only real decision. Most people do Zechawa first (Long Lake and Five-Color Pool) because it is the highest and gets crowded fastest, then Rize (Five-Flower Lake, Pearl Shoal, Primeval Forest) after lunch. The reverse also works and gives you Five-Flower Lake in better morning light.
Practical details
The park opens early — around 07:00 in peak season — and the last buses leave the upper valleys in the late afternoon. Do not leave your return to the last hour; the queues at Nuorilang can be long at closing time.
Food inside is limited to a few canteens and convenience counters, and it is expensive. Bring water and something to eat. Boardwalks are one-way in places and railings are there for a reason: the travertine is fragile and the fines for stepping off are enforced.