Abstract: The post-failure behavior of rock is analysed by a stability theory of system in this paper. It is pointed out that the stability of post-failure behavior of rock is related to the size and geometric parameter of specimen , elastic modulus and fracture energy of rock material, and testing condition such as confining pressure and stiffness of testing machine. To improve the control sensitivity is helpful to obtain complete load-displacement curve of rock specimen, but if can't change the stability of post-failure behavior. Adaptive control improve the control sensitivity, and don't change the stabilty of post-failure behavior of rock specimen.