When was the first large-scale southward migration of Han people in history?

The first large-scale southern construction was at the end of the Western Jin Dynasty, when the Five Wild Flowers were in China. After Liu Yao, the son of Liu Yuan, the Xiongnu leader, killed Emperor Jin, the Central Plains was completely occupied by the Hu people, and they burned and plundered the Han people everywhere, which was called "Five Wild Flowers" in history, also known as Lu Chen in the Central Plains. Since then, a large number of Han people have moved south. The Han people who moved south, headed by brothers Wang Dao and Wang Dun, established Si Marui as emperor and established the Eastern Jin Dynasty.

After the founding of the Eastern Jin Dynasty, people in the Central Plains were in dire straits under the rule of the Hu people for a long time without making progress and having no plans to recover the Central Plains. Later, King Ran was born to kill Hu and save the people in the Central Plains.