Abstract: A coattail catastrophe model of landslides was presented,and the necessary and sufficient conditions leading to landslides were discussed.The sliding surface of the landslides was assumed to be planar and the combination of two media: medium 1 was elastic-brittle or strain-hardening and medium 2 was strain-softening.In this way,a coattail catastrophe model of landslide system would be established.Through the analysis,it was discovered that the slope stability was greatly related with the stiffness ratio k of medium 1 to that at medium 2 and the geometrical-mechanical parameter ξ.Through analysis of parameters p,q and r.the complexity of the sliding system was found.When p was a fixed value,there were different zones of stability and instability in the control plane of q and r.