Abstract: A cusp catastrophe model is constructed for the strike-slip fault earthquake, a necessary criterion for judgement of sufficient mechanical earthquake and a simple expression of released energy by earthuake are presented. Based on the catastrophe theory analysis,a new assumption, simply called theory on induced earthquake by fault zone weakening and rockmass softening, is sugested reasonably explaining the relation between reservoir-induced earthquake and rock types,structures,water level,and the characteristic of b value of reservoir-induced earthguake sequence.We have a clear knowledge of reservoir-inuced earthquake mechanism from this paper discussion