Abstract: Although the new combat helmet can effectively reduce the pistol bullet penetrating damage, the back face deformation (BFD) of helmet may cause head injury. In order to accurately simulate the transient mechanical response of combat helmet under bullet impact, a progressive damage constitutive model for simulating the mechanical properties of composite combat helmet was developed based on the user material subroutine VUMAT of Abaqus. The finite element model of 9 mm lead core pistol bullet penetrating PASGT aramid combat helmet with impacting velocity 343 m/s was established. The accuracy of the numerical simulation was verified by the helmet BFD curve and the bulge shape of the inner surface. The failure mode of combat helmet shows that the helmet mainly occurs fiber tension, matrix compression and delamination failure. During the penetrating process, the stress contours on the helmet presents a regular diamond shape at the initial stage, and then slowly diffuses around and evolves into a circle. At three different angles of incidence (30°, 45°, 60°), the velocity of rebound is 72.9 m/s, 165.5 m/s and 240.1 m/s, respectively. Finally, the probability of skull fracture caused by the BFD of the helmet was estimated using the blunt criterion.