Abstract：According to the damage mechanics，the dynamic damage constitutive model of rock mass containing original damages is proposed. The formula of the average fragment size containing original damage variable after blasting is derived from the dynamic average fragmentation size formula of the fragile material given by Grady on the basis of fragmentation energy rules，fully considering the breaking action of the explosion stress wave and the motive fragment blocks extrusion and collision under the action of the detonation gas expansion and infiltration flow pressure. The experiment being made by our research group is aimed at verifying the results of the constitutive model. The results of the experiment coincide well with the calculation. The studies show that the influence of the original damages in the rock mass on blasting effect is very remarkable，and the average fragment size increase with the increasing of original damages. But the original damages can be controlled and utilized by modifying design parameters of blasting so as to improve the blasting effect. Some beneficial conclusions are summarized which are available for reference to practical projects.
