Abstract: A multi-modal elastic actuator is proposed based on the movement mechanism of lower limb muscles during human walking, and applied to knee exoskeleton to realize the rigid and soft drive of the knee exoskeleton mechanical leg. Firstly, the mechanism design of the multi-modal elastic actuator and knee exoskeleton is carried out, and the motion pattern of the actuator is analyzed. Then, the dynamic modeling of the actuator is carried out, and the influences of the elastic parameters, damping parameters and load on the output bandwidth of the actuator are analyzed. For the characteristics of the elastic actuator, the control strategy based on the motion state machine is studied. Finally, the knee exoskeleton prototype is controlled by the control strategy. The experimental results show that the knee exoskeleton can realize effective control according to the motion state machine, which verifies the rationality of the design of the elastic actuator.