Installing prosthesis is an important way of recover walking for amputees, which can improve the amputees' living and working ability. The structure of prosthetic knee joint has an important effect on prosthetic performance. Currently, most of prostheses products adopt passive damping structure to achieve similar movement to human knee by adjust damp. However, it cannot realize some gaits which require power for knee, such as upstairs or up the slope. In order to reduce energy consumption of the prosthesis, and improve gait adaptation, an active prostheses mechanism which contains spring is presented. A method is presented for analyze the impact of spring stiffness on energy consumption for the complex prosthesis dynamic system. Based on the presented method, dynamic analysis for the prosthetic is preceded, and the influence of spring stiffness on the driving power and driving energy is analyzed. Dynamic simulation is carried out for optimal design of prosthetic structure based on the minimum of energy consumption.