Northeast immigrants to the west.

China is densely populated in the southeast and sparsely populated in the northwest, roughly bounded by the Heihe-Tengchong line.

The reasons why China is not suitable for large-scale migration to the west are as follows: the western region is rugged and the cultivated land area is small, which is not suitable for large-scale agricultural development; Northwest China has a dry climate and water shortage, so it can't support more people, so it can't immigrate to the west on a large scale.