Objective：To Investigate the safety zone and indications of placing the upper sacral sacroiliac screw into the dysplastic sacra based on CT data. Methods：Totally 267 three-dimensional models of pelvis were reconstructed by Mimics（Materialize’s inter－active medical image control system） 16.0 software based on CT data and then the dysplastic sacra were detected. The virtual trans－verse screw which pass through the bilateral sacroiliac joint was planned to place into the dysplastic sacra，if not safe to be placed，the conventional unilateral sacroiliac screw was placed，and then the safety zone was measured. The skin around pelvic surface was recon－structed，and then the surface projections of the upper sacral central screw’s entry point P，anterior superior iliac spine’s vertex N，iliac crest point M were determined as P1，N1 and M1，respectively. The lengths of P1N1，P1M1 and M1N1 were measured in section. Results：Among 267 specimens，30.3 percent of sacra belonged to main dysplasia（upper sacral transverse sacroiliac screw could not be placed） and 9.0 percent of sacra belonged to minor dysplasia（upper sacral transverse sacroiliac screw could be placed）. For the main dysplastic sacra，the length of P1N1 was （162.52±21.23） mm in male and （163.52±20.39） mm in female（P=0.761）. The length of P1M1 was（129.29±17.38） mm in male and （111.56±17.84） mm in female（P=0.000）. The length of M1N1 was （146.92±11.08） mm in male and （146.72±15.05） mm in female（P=0.924）. The angle of the central screw oriented from posterior to anterior was （21.80±3.56）° in male and （19.97±3.02）° in female（P=0.000）. The angle oriented from caudal to cranial was （29.97±5.38）° in male and （28.15±6.21）° in female（P=0.047）. The length of the central screw in the Denis Ⅲ zone was （14.41±4.40） mm in male and （14.09±5.04） mm in female（P=0.665）；the length in the Denis Ⅱ and Ⅲ zones was （36.25±3.40） mm in male and （38.04±4.60） mm in female（P=0.005）. Conclusion：When the upper sacra have the feature of ‘the sacrum is not recessed within the pelvis’ and/or ‘the alar slope is acute’，the entry point of the upper sacral sacroiliac screw is more backward and caudal than the normal pelvic，and the angle oriented from posterior to ante－rior is about 20°，from caudal to cranial is about 30°. For the dysplastic sacra，the upper sacral sacroiliac screw is recommended to fix Denis Ⅰ zone sacral fracture and the sacroiliac dislocation. And there is no significant difference between man and women.