On the basis of catastrophic theory, taking bedding rock slope for example, a new catastrophic model on slope stability is established in consideration of elasto-brittle medium and strain-softening medium of the slip surface and the hydrostatic pressure of top splay fracture, then the mechanism of bedding rock slope stability is discussed. It is shown that: the stability of bedding rock slope is mainly affected by two factors, namely internal and external factors. The internal factors indicate that groundwater softens the rock mass of slip surface by physicochemical effect, so the rigidity ratio of rock mass is reduced, which plays a leading qualitative effect. The external factors refers to hydraulic pressure. It not only makes the sliding potential of the slope increasing, but also plays a role as a disturbance in the process of slope instability to speed up the landslide. On the basis of these, equations calculating the critical water height in the splay fracture and the critical rainfall intensity are derived, and then the bedding-slip failure hydraulic criterions for bedding rock slope are proposed. The displacement change rule of the rock mass of slip surface in the progressive softening is also given. The increasing of shear displacement is not uniform; it presents a leap which likes a ladder. Simultaneously, the physical model test is carried out (rigidity ratio ), which validates the rationality of instability mechanism and the displacement change rule of bedding rock slope with catastrophe theory. Furthermore, some suggestions of treatment technology and optimization methods for this kind of rock slope are proposed.