As early as the Kangxi period, before Emperor Kangxi recovered Taiwan Province Province, the policy of "sea ban" was implemented, and coastal residents were forbidden to get close to the sea, thus cutting off the connection between Taiwan Province Island and the mainland. Although the Qing Dynasty finally recovered Taiwan Province Province, it also made people's livelihood in coastal areas desolate and desolate. Fortunately, Emperor Kangxi loved western natural science and technology, and he opened many trading ports to do business with foreign countries, so at this time, the Qing Dynasty still had access to many western science and technology.

When Yong Zhengdi ascended the throne, there were only four coastal trading ports, which greatly reduced the communication with western countries. Although Yong Zhengdi also has a soft spot for western science and technology, the communication between China and the West is far from comparable to that of Kangxi Dynasty. During the Qianlong period, Emperor Qianlong stepped up his closed-door policy on the basis of Yong Zhengdi, and closely monitored and controlled western businessmen in China. By the end of Qianlong, there was only one foreign trade port in Guangzhou in the whole Qing Empire, which could only be sold under the strict control of the government.

The gap between China and the West is widening. Finally, in the Opium War 40 years after the death of Qianlong, the Qing Dynasty was defeated, which reminded people of the great relationship between the Opium War and the closed door. But at this time, it was too late, and the door opened by western powers lost its function of protecting "China going abroad" forever.