Generally speaking, there are three kinds of social security systems in the world, one is DB pay-as-you-go system, the other is DC full accumulation system, and finally there are various mixed models, such as semi-accumulation system and nominal account. DB Pay-as-you-go system The so-called DB Pay-as-you-go system refers to the "pay-as-you-go system", that is, the basic financing system of today's retired generation is paid immediately with the insurance premiums paid by the contemporary working generation. Generally speaking, there is a kind of savings called "social security fund". In order to reduce the pressure of payment, the central government invested it in some financial instruments, such as government bonds. Under this system, the calculation method of the pension payment standard of the retired generation mainly depends on the total salary level of the whole working life, especially in the last few years, as well as the comprehensive factors such as length of service, length of service, level and contribution.