The process of impact hammer drilling is very complex for high non-linearity, large deformation and damage behavior. The explicit finite element code ANSYS/LS-DYNA was employed to research the transient percussive characteristic of bit' inserts impacting brittle rock with different rate of air voids. The impact forces curves of concrete and sandstone under hammer impact has been recorded. From the simulation results, the rock void rate affects variously on the transient impact force of bit' inserts penetrating rock. The impact force between bit and rock decreases sharply as the rock void rate rises. However, the transient impact force between piston and bit has nothing with the rock void rate. The physical processes such as insert penetrating rocks subjected to percussive hammer drilling are displayed visually. It is believed that the numerical simulation will become a good research method for transient impact process.