Robot prosthesis has 6 degrees of freedom, including outward swinging and forward and backward swinging, which are droved by servo motors. In order to reduce the servo motors’ driving torque, a gas spring was added on shoulder to balance the gravity torque. In this paper, the difference between gravity torque and balanced torque was created and taken as optimized object firstly, and then a mathematical model was created and Matlab program on the base of nonlinear programming genetic algorithm were made for obtaining the optimum, lastly the simulation model and optimizing process diagrams were outputted.