The hydrodynamic load received by the rotary vane steering gear in the process of steering is complicated and it forms a strong coupling relationship with the steering angle and angular velocity,which seriously affects the control performance of the steering gear system.To solve the problem,on the basis of analyzing the fluid force on the surface of the rudder blade,a new structure decoupling scheme of the compound hydraulic swing cylinder was proposed.The compound swing cylinder had a double-layer structure,the rudder drive cylinder was nested inside the torque decoupling cylinder,the drive cylinder rotor was fixedly connected with the rudder stock,and the drive cylinder housing was doubled as the torque decoupling cylinder rotor.The steering gear angle was controlled by driving the driving cylinder rotor,the decoupling cylinder rotor was rotated,and the main torque was applied to the driving cylinder rotor to offset the load torque generated by the hydrodynamic force on the rudder stock,and to eliminate the influence of hydrodynamic force on the rudder angle control.The simulation result shows that using the compound swing cylinder,the interference of fluid force can be effectively solved,which is a qualitative help to the improvement of the quality of rudder angle control.