The numbers
The park entrance at Jiuzhaigou is around 2,000 metres; Shuzheng village 2,200; the Rize valley 2,500; Nuorilang 2,365; Long Lake, the highest stop on the bus route, 3,101. If you add Huanglong, the cable car tops out near 3,450 and the Five-Color Pool at the top of the walk is around 3,570, with the pass on the road between the two parks above 4,000 metres.
That is a moderate altitude profile. Most visitors feel nothing beyond breathlessness on stairs. A minority get a headache and a poor first night, and the risk rises sharply if you fly in from a lowland city and go straight up the valley.
How to pace it
Take the bus to the top of a valley and walk down, never the reverse. Jiuzhaigou's shuttle system makes this easy: ride to Long Lake or the Primeval Forest, then descend on the boardwalks and re-board when you have had enough. Climbing at 3,000 metres is what makes people feel unwell, and there is no reason to do it.
Drink more water than you think you need, eat breakfast, and skip alcohol on the first night. Move slowly for the first two hours at altitude — the temptation is to race between viewpoints before the crowds arrive, which is exactly the wrong thing.
Symptoms and what to do
Breathlessness on stairs, a mild headache and waking at night are normal at 3,000 metres and settle within a day. A headache that will not shift with painkillers, nausea, dizziness or confusion are not normal, and the answer is to descend — the park buses run constantly, and dropping 800 metres resolves most cases quickly.
Oxygen cans are sold at the park entrance and at the visitor centres, and there are oxygen bars in the towns. They help with a headache; they do not treat altitude illness. If someone in your group is genuinely unwell, get them down rather than treating them in place.
Who should think twice
Travellers with heart or lung disease, uncontrolled high blood pressure, or a previous bad reaction to altitude should get medical advice before booking, and should probably skip Huanglong. Otherwise the main precaution is simply not to treat the first afternoon as a race.