Rock failure process presents a characteristic of self organized criticality. A method which use self-organized extent parameter α to express stress transfer among units and stress concentration in local units is put forward. Based on the theory that the failure strength of rock units obey second power Weibull distribution, a mechanical model is established with renormalization group theory, and the critical probability of rock failure, pc, was derived. Results indicate that the failure of units is independent and stochastic, and the system evolves to a stable state when the failure probability of unit, p, is less than the critical failure probability pc. When p is higher than pc, the failure of units focus on the macroscopic transfixion tension split plane, which is a dynamic attractor of self organized criticality. The critical failure probability pc and the unit capacity of bearing external load F is related to initial condition. pc as well as F/F0, decreasing with the increase of self organized extent parameter α, and furthermore, F/F0
