Aiming at the failure problem of cased explosive charge under impact of kinetic energy penetrators, the velocity of case driven by explosive charge under different reaction rate was analyzed. The grade of characterizing explosive charge failure was proposed using equivalent gurney velocity or equivalent gurney energy. The method of solving equivalent gurney velocity and equivalent gurney energy was given accordingly. Based on equivalent gurney velocity, the failure grades of cased explosive charge were divided and the criterion was given ，the idea of deflagration failure is proposed which is considered to be more effective to realize. The deflagration failure problem of cased composition B under impact of kinetic energy penetrators was studied based on experimental tests, numerical simulation and analytical calculation. The relations between deflagration failure and normal detonation failure were analyzed and the lower limit value of deflagration failure criterion was given. The results shows that it can be regarded as the lower limit of deflagration failure of explosive when equivalent gurney velocity reaches 1% of gurney velocity.