An elastic actuator is proposed for meeting the effective drive demands of a lower limbs power-assisted exoskeleton under different walking patterns. The proposed actuator can drive a power-assisted knee exoskeleton to achieve different kinds of motion through driving the screw nut by a motor to connect a spring in series and combining with the corresponding brake pads. The motion mode of the elastic actuator is analyzed, and the dynamics of brake mechanism is studied. Some braking force and bounce impact experiments are implemented for selecting suitable brake pad material and spring. The effects of spring stiffness and equivalent mass on the elastic actuator properties are studied based on a virtual prototype co-simulation platform with SolidWorks and ADAMS. The simulation results provide theoretical basis for designing the elastic actuator of lower limb power-assisted robot.