The slope particles erosion under slope flow not only causes the serious soil erosion, but also the heavily gravity disaster, such as landslides and debris flows. So studying the internal mechanism of the erosion becomes a very important problem in disaster reduction and prevention project. In the article, on the basis of the formula of water dynamics and the view point of particles water environment on slope surface, the erosion mechanism of slope particles has been studied. Then, by analyzing the relationship between overland flow and slope erosion, the critical conditions of erosion for different kinds of particles are given. Finally, an example is given to prove the theory.