Modern vehicle safety design and safety regulations are mostly based on 50th percentile populations. However, with the increase of obese populations, it is very important to investigate the injury mechanism and protection of obese occupant. Methods such as traffic accidents statistics, cadaver experiments, multi-body modeling and finite element modeling, are currently used to study the injury mechanism of obese occupants. Different hypotheses including cushion effect, body geometrical effect and mass increasing effect have been put forward to explain the effect of obesity on occupant injury mechanism, which means that its mechanism is still uncertain. The impact injury mechanisms of obese occupant were comprehensively summarized. Furthermore, the problems confronted by the research of current obese occupant impact injury and future investigations were proposed in this study.