The twenty-two years of Qianlong (A.D. 1757) is the boundary of closed doors, which can be roughly divided into two different periods. The early rulers intended to isolate the communication between the mainland people and Zheng anti-Qing forces in Taiwan Province Province, resulting in people gathering at sea. However, after 1757, China closed its doors to the outside world with a view to preventing and prohibiting the "cross between the people and the foreign countries" and strictly prohibiting foreign trade in the form of treaty legislation.

I think the reason why Emperor Qianlong did this was entirely out of his conceit. Emperor Qianlong said in "Letter to the King of England": "China is rich in natural resources and everything. Originally, it didn't borrow foreign goods to make up for what it needed. " In the closed feudal natural economy, there was no need for communication and trade, but the rulers of the Qing Dynasty were proud of it and were arrogant. Due to the strict restrictions on overseas trade, it has seriously affected economic development and hindered social progress. At the same time, the people of China were isolated from the world trend, and the rulers of the Qing Dynasty turned a deaf ear to it. The gap between China and developed capitalist countries in science and technology is also widening, and China lags behind the world development trend after all.