In real ocean environment, direction and velocity of the tidal stream change frequently, therefore the output characteristic of the tidal turbines would be affected. Case study on a 50kW tidal turbine was carried out by numerical simulation. Based on the measured hydrological data of a certain observation point in the sea area of Zhaitang Island, the output of the turbine under time-varying flow conditions was analyzed by CFD to determine the turbine’s optimum installation orientation. Result of simulation of a tidal periodicity (15 days) showed that the power output of tidal turbine may be influenced by the change in the velocity and direction of tide and its installation orientation. The maximum power output would be reached if the turbine was installed facing the direction of about 62° (WGS-84) in current observation point, which was about 2500kWh in a tidal periodicity. Therefore, velocity asymmetry between flood and ebb tides should be fully considered to determine the installation orientation of the turbine, which provides references for the development of tidal energy in the actual marine environment.