In terms of the problem of strong potential coupling between rudder blade and hydrodynamic force in the motion of ship rotating blade steering gear, a new type of compound hydraulic oscillating cylinder steering gear is designed. In order to reduce the overall floor space of the structure, a multi-objective optimization model which takes the inner diameter of compound oscillating cylinder shell, inner diameter of torque decoupling cylinder rotor, diameter of driving cylinder rotor hub, height of moving blade and stator blade, width of moving blade and stator blade, and geometric dimensions of shell thickness as the design variables is established based on Multi-Objective Genetic Algorithm (MOGA). When Genetic Algorithm (GA) is used to solve the multivariable optimization problem under the nonlinear constraints, the fitness function is separated from the constraints by designing the penalty function to process the constraints, and the optimal dimension parameters are selected according to the set criteria. The stress-strain analysis of oscillating cylinder shell, torque decoupling cylinder rotor, and driving cylinder rotor is made with ANSYS. The results show that the structure dimension of compound oscillating cylinder is optimal on the premise of meeting the structure strength and stiffness, and safety factor, which verifies the reliability of optimized structure.