A crater blasting model of rock with pre-existing defects under blasting loads is established by using finite difference method. The defects are saturated with water and are randomly distributed. The defects include voids, pores and small open joints. A shock equation of state (EOS), obtained from the existing test results, is employed for the granite near the explosive charge, and for the granite far from the charge, a linear EOS is applied. A modified principal stress failure criterion is applied to determining the granite statuses. The granite dynamic strengths, which increase with strain rate, are calculated from the existing test results. The simulation results show that under blasting, the strength of wet rock is lower than that of dry rock.