Abstract: The genetic mechanism of the gently dipping structural plane in granite rock mass at Three-gorges damsite is systematically studied in the paper. Based on the field investigation, SEM and TEM analyses, and determination of inclusion temperature, the microstructural fabrics and differencial stress of rock mass at the damsite were studied in detail. With the geomechnical analysis, an inversion of data for genetic mechanism of gently dipping structural plane was made after the geomechanical physical simulation under similar conditions. Then using nonlinear FEM, the result was tested by numerical simulation. Such a comprehensive study in connection with geology, mechanics, microstructure and geomechanical, physical and numerical simulations is first carried out in the world, The result of study will satisfactory for requirements of engineering construction.