Abstract: On the basis of catastrophic theory ,a catastrophic model with cusp point for stratified rock mass with a gentle inclination is developed by using elastic mechanics theory .This model studies groundwater effect in a comprehensive way for the first time. Also, this model indicates that groundwater softens the rock mass of slide surface by the physicochemical effect and reduces rigidity ratio of the rock mass, and leads to sudden instability. Furthermore, the origin of landslide is discussed and some suggestions of harnessing the landslide are presented.