An elastic actuator is proposed for meeting the knee power-assisted exoskeleton effective drive demand under different walking pattern. The actuator can achieve different motion of power-assisted knee exoskeleton through the motor driving the screw nut, series with spring and combing with the corresponding brake pads, and the motion modes are analyzed. The kinematics equations of elastic actuator were established, and the effects of spring stiffness and damping for the properties of elastic actuator are studied. The prototype of actuator is designed and tested, and braking force and motion following experiments are implanted. The results of experiments showed that the elastic actuator can realize effective braking force and favorable motion following. The study of multi-mode elastic actuator provides important theoretical basis for the understanding of motion mechanism of bionic actuator.