Abstract: In order to solve the problem of accumulative landslide instability under rainfall,this paper presents a simple mechanical model considering the fissure water in trailing edge fracture of landslide. The landside system being at a quasi-static state in balance curved surface,the systems total potential energy function is established. So the dynamic instability cusp catastrophe model of the landslide system is generalized. When passing through the curve of fold bifurcations,the common equations of shear displacement jump、energy release and the slip bands velocity are obtained. The material constitutive relationship of Weibull distribution is introduced into the generalized fold model as a example. Through the fold bifurcation analysis of the cusp catastrophe,it is got that the landslide dynamic instability is the combined effect of internal and external results under rainfall. And this deepens the understanding of the mechanism of landslide instability.