This paper presents an electrical powered transfemoral prosthesis. The knee joint is actuated by a motor-driven ball screw assembly and the ankle joint is motioned by a series elastic actuator. An undirectional spring is attached paralleled with the SEA to store energy during dorsiflexion and bias load of ankle torque. To mimic biomechanical gait data and make the prosthesis compact and anthropomorphic, some key parameters are calculated to achieve optimal solution. These parameters include the length of crank rocker of knee and ankle joint, the stiffness of series and parallel spring and the dimension of the series leaf spring. The analysis procedure is demonstrated in this paper.