A two-dimension finite element model is established based on the simplified spine model for seated human body. The dynamical responses of human body are numerically calculated in different impact cases. The results show that the impact responses of the head and the head injury criterion(HIC) of the seated human body under vertical impact excitation are more serious than those under horizontal impact excitation, while the impact responses of waist under horizontal impact is larger than that under vertical impact. Though the safety belt at pelvis can protect human body to a certain extent, the safety protection of the safety belt at shoulders is more effective. It is found that using the safety belts at shoulders and pelvis at the same time, the impact response of human body and the HIC can be decreased to a large extent and it is a promising protection measure for seated human body in vehicles.