The judgement formula for high and low speed landslide and a new theory of stiffness - effect instability are presented for the instability of planar - sliding slope by the catastrophe theory . it is pointed out that the limit equilibrium stability analysis method of rigidity body has a lot of defects . the relationship between the action of outside environment and the response of slope system is complicatedly nonlinear according to the nonlinear dynamics model of slope evolution built 针对斜坡平面滑动失稳问题，运用突变理论方法，给出了快速滑坡和慢速滑坡发生的判据，验证了刚体极限平衡稳定性评价方法有较大缺陷；根据斜坡演化非线性动力学模型，发现斜坡演化过程会出现混沌现象，其通向混沌之路是通过倍周期分叉实现的。
