For tidal power stations, turbines operate in a wide head range and sometimes near the free surface due to tide changes, which commonly consists of short intake channel. When they operate under low water levels, free surface flows would be a major concern for hydraulic stability. The present paper establishes a simulation method for predicting all flow fields in tidal power stations. Two-phase simulation model is introduced with the acceleration of gravity and the free surface, which consists of a turbine unit and two reservoir models (sea side and reservoir side). Hydraulic performance of a prototype turbine was predicted by using the proposed model, based on water levels of both two sides. The predicted unit performance agreed well with the test result. The flow fields were compared with ones by two other simulation methods. The proposed method could predict non-uniform intake flow inducing head losses by the sudden contraction and the gravity force, which sufficiently influenced turbine internal flows. Consequently, head drops in each part of the turbine were distributed differently, under the same net head. The present model provides a foundation for predicting prototype turbine performances under not only tide changes but also air-water interactions.