When was the first great migration in Chinese history?

The first time happened in the late Western Jin Dynasty.

Ten years after the Jin Dynasty unified China, the "Eight Kings Rebellion" broke out, and the national strength was greatly damaged. By the end of the Western Jin Dynasty, the water conservancy in the Central Plains was in disrepair, and droughts, locusts and diseases continued year after year, coupled with the impact of Wuhu forces in the west and north, resulting in a large-scale immigration tide. The population of the Yellow River basin moved to the Yangtze River basin, and the population of the Yangtze River basin moved further south, forming the first great population migration in ancient China. The population moving south this time is about 900,000, which has changed the population distribution pattern of more in the north and less in the south since Qin and Han dynasties, and the population in the south has increased rapidly, which is a sign that the population distribution center in China has shifted to the Yangtze River basin.