Abstract: A coattail catastrophe model is presented.The necessary and sufficient conditions leading to slope failure are discussed.The sliding belt is assumed to be planar.The sliding belt and the upside crag body are composed of the different materials with different mechanical properties.The upside crag body is modeled as the elastic brittle material（called medium 1）;the sliding belt medium is modeled as a strain-softening material（called medium 2）.The shear stress-strain constitutive model is described by the Weibull's distribution law.In this way,a coattail catastrophe model is developed for the slope failures.Through the analysis,it is discovered that the slope steadily has a good relation with the stiffness ratio of medium 1 to medium 2.Along with increase,the 0 value that slope starts to skid reduces,and the value * of skips also reduces.At the same time,the marginal value x* increases,and the skip quantitative x changes to be smaller.