In this study, we present the principle design of a fully-passive transfemoral prosthesis for normal walking, inspired by the power flow in human natural gait. The working principle of the mechanism is based on three parts, which are responsible of the energetic coupling between the knee and ankle joints. The design parameters of the prosthesis have been determined according to the energy absorption intervals of the natural human gait. Simulation results show that significant amount of energy can be stored to deliver for ankle push-off generation. The CAD representation of the prototype that is under tests is also presented.