How serious was the population decline in eastern Sichuan in the late Ming and early Qing dynasties?

The most direct reason is the massacre of Sichuan population by Zhang, Wu Sangui and others.

Huguang's filling in Sichuan refers to a large-scale migration in the Qing Dynasty. According to research, residents of more than a dozen provinces such as Jiangxi, Fujian and Guangxi are also among the immigrants. At the end of Yuan Dynasty, Ming Dynasty and Qing Dynasty, Sichuan experienced wars and its population dropped sharply. Therefore, a series of measures have been taken from the central government to the local governments to attract immigrants, among which Huguangxing Province has the largest population. Taking Chengdu as an example, the Overview of Chengdu in the late Qing Dynasty recorded that "Chengdu people today are all from other provinces"; Among them, Huguang accounts for 25%, Henan Shandong 5%, Shaanxi 10%, Yunnan Guizhou 15%, Jiangxi 15%, Anhui 5%, Jiangsu and Zhejiang 10%, Guangdong Guangxi 10%, and Fujian.