Abstract: In order to realize the yaw hydrodynamic performance monitoring of passively rotating turbine, DFBI （dynamic fluid body interaction） simulation method based on overlap grid is used in this paper, the influence of hydrodynamic change of flow direction on yaw hydrodynamic performance of marine current turbine, and put forward stern rudder in order to optimize turbine structure, make a contrastive study of different stern rudder. The result shows that the change of flow direction led to the energy efficiency drop, installation of stern rudder can realize the turbine runs in the direction of the flow, and effectively improving the hydrodynamic performance of turbine. In low velocity environment, the rudder which area is 10% of flow area of turbine can improve the hydrodynamic performance of turbine greatly, it is suitable for the rudder of turbine.