In order to solve the needs of the disabled for limb rehabilitation, the prosthetic knee joint can swing freely with the intention of the stump user. According to the driving principle of the prosthetic knee joint of MR Damper, the prosthetic limb is designed. By making full use of the magnetization characteristics of magnetorheological fluid, the magnitude of current is related to the magnitude of damping force, and the torque of the knee joint is changed by changing the input current to control the swing of the prosthetic knee joint and improve the movement flexibility of the prosthetic knee joint. The results show that the proposed prosthetic based on MR Damper can be passively bent and actively extended during horizontal walking swing, and has good stability and flexibility.