Abstract: In order to solve the problems on similitude law existing in simulative experiment for controlling hydraulic erosion in small watershed of Loess Plateau in China, the authors analyze the characteristics of hydraulic erosion in small watershed of Loess Plateau, give the basic similitude criteria and scale expressions for physically simulative experiment according to principles of hydromechanics and similarity theory. The results indicated that the basic water movement is turbulent flow and follows sediment-transporting equations of turbulent flow. The simulative experiment should obey not only geometric similarity, but also kinematical and dynamic similarity. The basic similarity scale expressions are the ones of the geometric, rainfall, flow, erosion sediment transport, bed deformation and soil water movement similarity etc.