What period does Yiguan Nandu refer to?

The late Western Jin Dynasty.

During the Western Jin Dynasty, large-scale wars continued in the Central Plains during the reigns of Emperor Huai and Emperor Min of Jin. The surrounding tribes that migrated inward established monarchical regimes one after another. They became stronger and threatened the Western Jin regime, and eventually led to the disaster of Yongjia. The central court threatened.

During the Jianwu period of the Jin Dynasty, a large number of gentry, scholar-bureaucrats, and common people moved southward. This was known in history as the "Yongjia Rebellion, and the Southern Crossing of Clothes". This was the first large-scale southward migration of Han people from the Central Plains. After the Western Jin Dynasty crossed the river, it established its capital in Jiankang (now Nanjing), which was called the Eastern Jin Dynasty in history.

Extended information:

After the Han Dynasty, many great chaos and foreign invasions caused a large number of northerners to immigrate to the south of the Yangtze River. After the Yongjia Rebellion, the scale of immigration in the Central Plains increased.

The first route was to move southward, crossing the Huaihe River to the middle and lower reaches of the Yangtze River, which brought about the southward shift of the political and economic center of gravity and the southward expansion of culture. The second route was to the eastern Liaoning region, defecting to Wang Jun who was entrenched in Youzhou at that time, Cui Se, the governor of Pingzhou, and the Xianbei Duan tribe and Murong tribe in western Liaoning.

Murong Xun specially established Jiyang County, Chengzhou County, Yingqiu County, and Tangguo County to resettle the refugees in Jizhou, Yuzhou, Qingzhou, Bingzhou and other places respectively. In addition, there are areas from the Central Plains to Bingzhou in the north and Jingxiang area in the south.

These spontaneous immigrants, coupled with large-scale and frequent government-forced immigration due to wars and other events, not only promoted ethnic integration, but also expanded China's cultural frontier.

Baidu Encyclopedia - Yiguan Nandu