Objective    To study the mechanics of pelvis under extorsion loading using three-dimensional finite element model (FEM).　Methods　A healthy adult male volunteer was chosen to undergo PET-CT scanning. The initial images of pelvis were output to PC. Next a three-dimensional reconstruction and a finite element model were made. An extorsion loading was put to the model. The horizontal loading reached to the left anterosuperior iliac spine posteriorly. Then the stress, strain and displacement nephograms were obtained and the biomechanical features were analyzed.　Results　The stress conducted through the two ways. Firstly, it was rearwards to the superior part of the sacrum passed the sacroilial joint at the same side. Secondly, the stress passed forward to the pubis at the opposite side. The loading of the anterior ring of pelvis was relatively greater than that of other parts. The peak value of stain appeared at the antero-inferior part of the sacroilial joint at the same side. The peak value of displacement was at the loading point. The displacement at ilium, ischium and ramus of pubis at the same side were relatively greater. Conclusions　The extorsion stress easily cause the fractures of pubic branch fracture, the damage of sacroilial joint, or the compressed fracture of sacrum.