A new type of ankle prosthesis with lower power combining active and passive actuators is designed based on the research results of human ankle joint biomechanics. The human motion during level walking with slow speed is divided into four phases by analyzing the relation of ankle joint torque versus angle, the idealization and decomposition of walking movement, which are the plantarflexion after heel-strike buffering, the dorsiflexion during body forward, the plantarflexion during pushing off ground and swing. A series elastic actuator system and a parallel spring are used to produce walking motion, and the ankle prosthesis is designed with a series elastic actuator and parallel spring. The motion simulation of ankle prosthesis is implemented with SolidWorks and ADAMS softwares. The results show that the ankle prosthesis can realize the expected movement trajectory, which verifies the correctness of the design.