Why is the proportion of Han nationality in Qaidam Basin so high?

Because the population of Qaidam basin is mainly concentrated in towns, especially Golmud.

Most towns are basically immigrant cities, and they are all Han people who immigrated from the mainland decades ago, so the ethnic composition is mainly Han. However, with the strict family planning for the Han nationality and the free birth of other local ethnic minorities such as the Hui nationality, the proportion of the Han nationality is gradually declining.

As for the altitude of Qaidam, it is basically around 2000-3000 meters. The whole Qinghai-Tibet Plateau belongs to a relatively low altitude area, and the plateau reflection is far less than that in most parts of Tibet and Qinghai.