A four-link hydraulic buffer knee joint assembly which includes a hydraulic center axle configured at a center hole of a four-link hydraulic main body and utilizes a spring force and an oil-filling piston to achieve the auto-filling of hydraulic oil. The hydraulic center axle is able to utilize adjustable knobs provided on an outer side of the center axle to adjust the bending speed and the spring-back recovery speed. The setting location of the hydraulic center axle differs from the one of conventional joint assembly such that the overall size of the knee joint assembly is reduced while the joint assembly includes the functions of adjustable hydraulic bending and stretching speeds and buffering for the hydraulic head to return to its horizontal state. The four-link hydraulic main body comprises a setting changeable controlling axle for achieving the function of the buffered safety brake against the ground.