By using finite element method and establishing a generalized model for the classical slope-gully system of Uuandigou watershed,it is found that with gradual rise of dam land,the stability of the slope-gully system is enhanced and the slumping amount is reduced,which separately satisfies binomial and linear rules.Through calculating and simulating by software,the erosion amounts and percentages of gravitational erosion on mound and gully slopes are obtained.For gravitational erosion of the slope-gully system，with dam land rising gradually,ratio of the volume to weight of slumping soil on mound slope or gully slope is about 1:50.In addition,the volume and weight of slumping soil on mound slope are about 2 percent of the volume and weight of the total slumping soil,respectively,while on gully slope,about 98 percent.Thus,the gravitational erosion on gully slope is the main source of channel sediment. Result from the study can provide important parameters and a scientific foundation for predicting and quantitatively evaluating gravitational erolion in small watershed.