The penetration of long-rod projectiles into semi-infinite concrete was divided into crater, penetration during eroding state and penetration during rigid state. The mass loss of projectiles was calculated in the eroding state while the increment of penetration of projectile was calculated in the penetration state. The total penetration depth during eroding state was obtained by summing the increment of penetration. When the projectiles penetrated concrete at rigid state, the penetration depth of projectiles was calculated based on cavity expansion theory. Penetration depth of tungsten alloy eroding projectiles into semi-infinite concrete was obtained by this analysis model and it was identical to the result obtained by the experiment.