In view of the strong coupling between rudder blade and hydrodynamic force in the course of the movement of the ship rotor rudder, a new type of double hydraulic swing cylinder steering mechanism is designed. In order to reduce the overall floor area of the structure, firstly, a multi-objective optimization model was established based on the multi-objective genetic algorithm with the geometric dimensions of the shell inner diameter of the compound swing cylinder, the rotor inner diameter of the torque decoupling cylinder, the rotor hub diameter of the driving cylinder, the height of the moving and static blades, the width of the moving and static blades and the shell thickness as the design variables, When genetic algorithm is used to solve multivariable optimization problems with nonlinear constraints, penalty function is designed to separate fitness function from constraints to deal with constraints, and the optimal size parameters are selected according to the set criteria. Finally, the stress-strain analysis of the shell, torque decoupling cylinder and drive cylinder rotor is carried out by ANSYS. The results show that the structure size of the double swing cylinder is the best under the premise of satisfying the structural strength, rigidity and safety coefficient, and the reliability of the optimized structure is verified.