Abstract: Based on the simple mechanical model of large dam deformation,the cusp catastrophe model of large dam destabilization is established by the catastrophe theory in this paper. The nonlinear strain and water-permeating weakening of the crack of the dam are taken into consideration. The mechanism of deformation and destabilization process of large dam as well as it's force condition are qualitively analyzed. The discussion presented in this paper may be helpful to wholely understanding the mechanism of large dam destabilization.