Objective To establish a valid human head-neck model and ballistic helmet model, and analyze biomechanical responses of the cervical spine under bullet impacts on ballistic helmet with different weights. Methods A uniformly distributed weight of 2 kg was applied on the helmet (1.24 kg), and bullet impacts from frontal, lateral and crown directions at the speed of 450 m/s were considered to obtain the mechanical response of human vertebrae. Results The stress of the cervical spine was significantly higher than that of the skull under bullet impacts, and the stress of C3 segment was the largest, indicating that the cervical spine was more vulnerable than the head during bullet impacts under the protection of ballistic helmet. When the weight of helmet attachment was not considered, the maximum stress of the cervical spine under lateral impact was 2.58% higher than that under frontal and crown impacts. The frontal impact led to the greatest damage to the head, with an increase of 59.4% in head stress. When the weight of helmet attachment was considered, a lager helmet weight would cause a more serious spine injure. When the helmet weight was increased from 1.24 kg to 3.24 kg, the crown impact led to the greatest damage to the cervical spine, with an increase of 12.98% in cervical stress compared with impacts from other directions. Conclusions Lightweight should be considered in the design of ballistic helmet, and the research findings provide scientific references for the design of ballistic helmet.