Abstract: In order to study the transient mechanical response of the ballistic helmet in the case of non-penetrating and the severity of behind helmet blunt trauma (BHBT), an experimental study was carried out using 3D-digital image correlation (3D-DIC). The dynamic back face deformation (BFD) of the helmet, the deformation speed and the deformation area during loading were obtained. Based on these results, the blunt criterion (BC) and the abbreviated injury scale (AIS) were adopted to evaluate the degree of BHBT. The results show that the average maximum height of BFD is 27.8 mm, and the average static height of BFD is 9.5 mm. The gap between the helmet and the skull has a significant effect on the severity of BHBT. When the gap is 12.7 mm, the average AIS score is 5, and the average skull fracture probability is 22.3%; when the gap is 15 mm, the average AIS score is 2%, and the average skull fracture probability is 3.8%.