Objective The injury mechanisms of three-year-old child occupants was investigated by reconstructing the real traffic accident. Method A traffic accident case from the CIREN database was reconstructed by using the vehicle finite element model and the three-year-old child occupant injury bionic model (TUST IBMs 3YO-O). The delta V, the mass of the vehicle and the deformation energy were comprehensively analyzed to figure out the collision velocity of the vehicles. This accident was simulated to present the injuries of the child occupant as described by the doctor. The injury mechanisms were analyzed in depth. Results The TUST IBMs 3YO-O fully reproduces the injuries to child occupant in the case. And on this basis, the assessment parameters and injury thresholds of child occupant used widely at present were analyzed. The kinematic response and biomechanical response of child head exhibited the difference. The neck injury of child was consistent with the description in the case. The biomechanical response of internal tissues and organs in the chest cavity showed that there was no injury, but the 3ms chest resultant acceleration exceeded the threshold. Conclusion In the future, it is necessary to enhance the evaluation of the biomechanical model. The human biomechanical models with high biofidelity applied to reconstruct the occupant injuries in traffic accidents are not only used to observe the kinematic responses of the occupant in the accident and an in-depth analysis of the injury mechanisms, but also provides the reference data for virtual testing. This will be helpful for the research and development of child occupant protection devices and the formulation of safety regulations.