Based on the self-organized criticality theory and OPF with transient stability constraints（OTS）， a new cascading failure and blackout model is proposed， which contains three different time scale dynamics， i.e.， transient dynamics， power flow dynamics and power grid growth dynamics. The cascading failure and the characteristics of self-organized criticality in power network evolution are investigated in this model. The stability of the transient dynamics is evaluated with the transient stability margin index based on the stability region boundary. By this index， the differential-algebraic equation type transient stability constraints are converted to algebraic constraints with the aid of sensitivities. Furthermore， appropriate preventive control（OTS） and emergency control（generator tripping and load shedding） against the random potential faults are employed to simulate the transient dynamics. The effectiveness of the blackout model proposed is verified by the simulation results concerning a New England system.