Abstract: The limit equilibrium equation for the analysis of earth slope stability is derived from the system energy by the use of the stress softening model and the instability is analyzed by the catastrophetheory. It is indicated that the catastrophic instability sliding arisesaccompanying an abrupt increment of the sliding-displacement when theslope parameter reaches the critical value. The limit equilibriumstability theory of a rigid body can not explain the catastrophe ofinstability of the stress softening earth slope.