What to wear to Jiuzhaigou Huanglong Scenic Area on May Day, is it cold?

The altitude is not too high, just wear a thicker coat on May Day. If you encounter a sharp drop in temperature, you should be more prepared.

Consider a down jacket just in case

You need to wear a down jacket, the plateau climate, the temperature difference between morning and evening is relatively large

It depends on the weather. The temperature difference between morning and evening is large. If it is not rainy, you should not bring a down jacket. It is still necessary to bring a thick coat.