In this paper, an actuator module for the knee joint of the exoskeleton for paraplegics, WalkON Suit4.0 is proposed. Proposed actuation module comprises of a BLDC motor, a cycloidal reducer, and an elastic component which are connected in presented order. This actuator module is optimized in assisting knee joint motion of a human. Assisting torque of 110Nm is realizable from high gear ratio of the cycloidal reducer and an additional torque of around 71Nm can be generated by the parallel elastic components. This additional torque is generated within the target angle range located near the end of the knee flexion angle. As a result, motions that require less torque but large speed such as walking and those that require large torque but less speed such as knee extension while ascending the stairs or slope can both be accomplished with proposed knee actuator module.