Objective To investigate the injury mechanisms of three-year-old child occupants by reconstructing a real traffic accident. Methods A traffic accident case from the CIREN database was reconstructed using a vehicle finite element model and a three-year-old child occupant injury bionic model (TUST IBMs 3YO-O). The Δv, mass of the vehicle, and deformation energy were comprehensively analyzed to calculate the collision velocity of the vehicle. This accident was simulated to present injuries to a child occupant, and the injury mechanisms were analyzed in depth. Results The TUST IBMs 3YO-O fully reconstructed the injuries of the child occupant in this case. The kinematic and biomechanical responses of the children's heads differed. The biomechanical response of the internal tissues and organs in the chest cavity showed no injury, however, the resultant chest acceleration at 3 ms reached 54 g, which exceeded the threshold. Conclusions In the future, it will be necessary to adopt biomechanical parameters for occupant safety evaluations. The application of human biomechanical models with high biofidelity to reconstruct occupant injuries in traffic accidents can not only be used to observe the kinematic responses of the occupant in the accident and analyze the injury mechanisms in depth, but also to provide references for virtual testing, as well as for the research and development of child occupant protection devices and the formulation of safety regulations.