Aiming at the problem that the hydrodynamic load received by the rotary vane steering gear in the process of steering is complicated and it forms a strong position coupling relationship with the steering angle and angular velocity, which seriously affects the control performance of the steering gear system. Based on the analysis of the fluid force on the surface of the rudder blade, a new structure decoupling scheme of the compound hydraulic swing cylinder is proposed. The compound hydraulic swing cylinder is a double-layer structure, the inner layer is the rotor of the driving cylinder, and the hollow structure is adopted, which is assembled with the rudder stock through the hole shaft. The outer layer is the rotor of the torque decoupling cylinder, which is also used as the shell of the driving cylinder. The rotor of the driving cylinder is nested in the rotor of the decoupling cylinder to control the speed and angle of steering gear. The rotor of the decoupling cylinder and the rotor of the driving cylinder rotate in the same direction, and the active torque is applied on the rotor of the driving cylinder to counteract the interference of the hydrodynamic load caused by the rotation of the rudder blade on the motion posture of the rotor of the driving cylinder, so as to eliminate the influence of the hydrodynamic force on the control of the rudder angle. The simulation analysis and experiment show that the structure decoupling of compound hydraulic swing cylinder is effective.