Abstract: A rigid-perfectly plastic model in incompressible medium is used in this paper. By setting a dynamical admissible velocity field, the resistance of medium to a flat-nosed block and a conical-nosed block are analyzed with an upper-bound way of limit analysis theory respectively, and the resistance formulae are simplified by numerical analysis. According to the characteristics of resistance curves, the critical thickness conditions and calculating theory of spalling and penetration are represented and the approximate calculation formulae under the impact of a flat-nosed block and a conical-nosed block with physical bases are obtained respectively, and a unified formula with the shape factor of blocks is integrated. The reliability and practicality of the method is verified by the comparison with experimental formulae.