Application of hydraulic transmission in tidal current energy conversion system is a new exploration. A stand-alone variable-speed constant-frequency tidal current energy conversion system based on hydraulic transmission is proposed. The system components and working principle are introduced first followed by the theoretical analysis of the system. Based on the adjustment of the pump displacement， the load size， and the motor displacement， a regulation and control principle for the variable speed， constant frequency and pressure operation of the system is proposed. A simulation model is established for the whole system， and a series of tests are conducted with and without controllers， respectively. The simulation results show that with the proposed regulation and control principle， the variable speed， constant frequency and pressure operation of the hydraulic tidal energy system can be achieved. This verifies the correctness of the theoretical analysis and provides theoretical basis for the design and tests of the actual system. Furthermore， the run-in tests of the prototype system based on a motor driving test bed have verified the feasibility of the application of the hydraulic transmission to the tidal current energy systems.