Providing powered joint actuation is a major focus of research in the lower-limb prosthesis area. The capability of actively powering the joints enables the prosthesis to meet the energetic requirements of locomotion, and thus provide higher performance in restoring the lost lower-limb functions in comparison with traditional passive prostheses. In this paper, a powered transfemoral (above-knee) prosthesis is presented, in which the knee and ankle are powered with pneumatic muscle actuators. A new variable-radius pulley-based mechanism is utilized, which enables the free adjustment of actuation torque curve to better match the desired torque curve as dictated by the locomotive requirements. Additionally, a spring-return mechanism is also incorporated, which replaces the muscle actuator in the “weak” (less torque-demanding) direction with a set of mechanical springs. With this mechanism, both knee and ankle joints can be powered while maintaining a compact profile of the prosthesis. The design details are presented, and the prosthesis is able to provide sufficient torque for an 85 kg user in various locomotion modes. A prototype of the prosthesis has been fabricated and tested, with the preliminary results indicating that this prosthesis is able to provide a walking gait similar to that of a healthy person.