Projectiles were regarded as rigid ones to simplify the mechanics principle of the perforation for concrete targets. According to the hydrodynamics model, the cracks were considered to increase astatically when they expanded to the back of the target under the impact of a projectile. This time was defined as the moment that the perforating action began. The critical distance to the back of the target was deduced by the energy dissipation mechanism for crack extension. After the cracks arrived at the backs of the targets, the penetration resistance was calculated by considering the relative velocity between the projectile and the plug. The calculated result indicates that the deceleration changes remarkably when the warhead is intruding the target and after the perforating action begins, whereas the deceleration changes weakly between the above two stages. Comparison between the calculated results and the existent experimental results shows the calculated results are credible.