In light of the problem of speed control for the tidal current turbine，the volume control method is adopted to control the impeller speed and the generator speed of the stand-alone tidal current turbine.Two similar hydraulic transmission systems for the tidal current turbine are designed，both adopting the pump-motor-generator direct connecting structure.To validate the hydraulic transmission systems，the mathematical models for the systems are analyzed and the MATLAB/Simulink models are developed to study the working characteristics of the hydraulic transmission systems.The results show that both hydraulic transmission topologies can realize constant-frequency control of the generator on the premise of meeting the requirement of maximum power point tracking control.