The relationship between the deformation and stability of the ancient landslide is studied in this paper. The necessity of considering the stiffness degradation in the process of reducing the mechanical parameters of the landslide is proved by the formula derivation, and a new method that considers the coupling effect of stiffness- and strength- degradation is presented. The deformation curve of the instability in the ancient landslide is drawn by a large number of engineering experiences, and the concepts about the warning displacement value of self-stabilizing ability and the ultimate displacement value of ancient landslide are put forward and defined by a determined method. Comparing with the methods of global strength degradation and slide zone strength degradation and the method in this paper applied to actual engineering example, it is seen that the method in this paper is more realistic to analyze the deformation field of the ancient landslide.