This research examined the influence of the change of the mass of each part of a human body on pelvis injury using the compression between the right and left acetabulum of the human FE model, which was identified as the best predictor of pubic rami fracture by a previous research. Four vehicle models were used to represent different loading locations from the hood edge. The sensitivity of the mass of each part of a human body to the pelvis injury index was analyzed by changing the mass density of the following parts to 1/10:​ 1) contralateral thigh and leg flesh, 2) contralateral leg flesh, 3) struck side thigh and leg flesh, 4) struck side leg flesh. In addition, 5) the mass of the upper body was reduced to 1/10. The changes of the peak values of the pelvis injury index were investigated for the combinations of these cases and the four vehicles. The mechanism behind these differences was discussed by investigating time histories of the pelvis injury index, pelvis and lower limb internal loads. In addition, the assumptions of the mechanism were developed and validated using a simplified pedestrian model that represents the mechanism in a more simplistic manner than that of the actual human.