Based on the experiments of cylindrical flat nosed copper projectiles impacting Comp B with various thickness steel cover plates,the explosion processes of cylindrical flat and round nosed tungsten projectiles impacting Comp B with various thickness steel cover plates were simulated. The curves of critical velocity vs thickness of the steel cover plate were quantitatively analyzed. The results show that the calculational results of direct shock initiation satisfy Jacobs criterion to a larger extent. Furthermore,the relationship between flat and round nosed tungsten projectiles velocities were obtained,i.e. V_round ≈1.15V_flat. At last,the reason of the delayed detonation(XDT) events was analysed. The cause was attributed to the mutual effects between the damage of Comp B and the reflection shock.