In the 1990s, China imported two 877ekm and two 636 submarines from Russia at a price of US $1billion. The unit price of each submarine was more than US $200 million. After a period of use, the Chinese navy is very satisfied with its performance. So in 2002, eight 636m "kilo" class submarines were added.So far, the Chinese navy has owned 12 kilo class submarines, becoming the largest overseas user of kilo class conventional power submarines. At the same time, unlike other foreign users of "kilo" class submarines such as India and Vietnam, China introduced relevant maintenance equipment and technology when importing the earliest 877ekm submarine.Besides Russia, China has also become the only country in the world with "kilo" class submarine maintenance capability.It has been in service for nearly 30 years. The early 877ekm "kilo" class submarine was once the "strongest conventional submarine" of our army;From the late 1980s to the early 1990s, the overall equipment status of the Chinese navy was not good. More or less, the surface fleet still has new projects effectively advancing. The 039 "song" class submarines are still in constant adjustment, and there is still a long way to go before mass production. The Chinese Navy's conventional submarine force has only a large number of obsolete 033 and 035 class old submarines.The best way to improve the current situation of the Chinese Navy's conventional submarine force and enhance the Chinese Navy's underwater combat effectiveness in a short time is to introduce mature advanced conventional powered submarines from abroad. At this time, Russia, with a weak economy, also urgently needs to "recover blood" through arms sales. So in 1992, the Chinese Navy began to visit Russia with the intention of purchasing "kilo" class submarines.After investigation, the Chinese navy was very satisfied with the performance of the kilo class submarine and decided to introduce it. Considering that the Chinese navy is in urgent need of modern