The knee joint with adequate torque is necessary in order to achieve natural, fast and versatile motions. However, the margin of torque for a knee joint occurs because of a reducer with constant reduction ratio. Therefore, a new rigid mechanism for a knee joint with continuously variable reduction ratio is proposed in this paper. Firstly, the requirements of a new mechanism should be met to reduce the margin of torque. And the requirements were got based on motion simulation of BHR-5. Then, the new mechanism is developed and the parameters of link length and the angle between links for the new mechanism is determined. Finally, the motion simulation of BHR-5 based on the new mechanism is conducted to verify the effectiveness and feasibility of the new rigid mechanism.