Objective To compare the biomechanical differences among the five internal fixation modes in treatment of Day type Ⅱ crescent fracture dislocation of pelvis (CFDP), and find an internal fixation mode which was the most consistent with mechanical principles. Methods Based on the pelvic CT data of a healthy adult male volunteer, a Day type Ⅱ CFDP finite element model was established by using Mimics 17.0, ANSYS 12.0-ICEM, Abaqus 2020, and SolidWorks 2012 softwares. After verifying the validity of the finite element model by comparing the anatomical parameters with the three-dimensional reconstruction model and the mechanical validity verification, the fracture and dislocated joint of models were fixed with S1 sacroiliac screw combined with 1 LC-Ⅱ screw (S1+LC-Ⅱ group), S1 sacroiliac screw combined with 2 LC-Ⅱ screws (S1+2LC-Ⅱ group), S1 sacroiliac screw combined with 2 posterior iliac screws (S1+2PIS group), S1 and S2 sacroiliac screws combined with 1 LC-Ⅱ screw (S1+S2+LC-Ⅱ group), S2-alar-iliac (S2AI) screw combined with 1 LC-Ⅱ screw (S2AI+LC-Ⅱ group), respectively. After each internal fixation model was loaded with a force of 600 N in the standing position, the maximum displacement of the crescent fracture fragments, the maximum stress of the internal fixation (the maximum stress of the screw at the ilium fracture and the maximum stress of the screw at the sacroiliac joint), sacroiliac joint displacement, and bone stress distribution around internal fixation were observed in 5 groups. Results The finite element model in this study has been verified to be effective. After loading 600 N stress, there was a certain displacement of the crescent fracture of pelvis in each internal fixation model, among which the S1+LC-Ⅱ group was the largest, the S1+2LC-Ⅱ group and the S1+2PIS group were the smallest. The maximum stress of the internal fixation mainly concentrated at the sacroiliac joint and the fracture line of crescent fracture. The maximum stress of the screw at the sacroiliac joint was the largest in the S1+LC-Ⅱ group and the smallest in the S2AI+LC-Ⅱ group. The maximum stress of the screw at the ilium fracture was the largest in the S1+2PIS group and the smallest in the S1+2LC-Ⅱ group. The displacement of the sacroiliac joint was the largest in the S1+LC-Ⅱ group and the smallest in the S1+S2+LC-Ⅱ group. In each internal fixation model, the maximum stress around the sacroiliac screws concentrated on the contact surface between the screw and the cortical bone, the maximum stress around the screws at the iliac bone concentrated on the cancellous bone of the fracture line, and the maximum stress around the S2AI screw concentrated on the cancellous bone on the iliac side. The maximum bone stress around the screws at the sacroiliac joint was the largest in the S1+LC-Ⅱ group and the smallest in the S2AI+LC-Ⅱ group. The maximum bone stress around the screws at the ilium was the largest in the S1+2PIS group and the smallest in the S1+LC-Ⅱ group. Conclusion For the treatment of Day type Ⅱ CFDP, it is recommended to choose S1 sacroiliac screw combined with 1 LC-Ⅱ screw for internal fixation, which can achieve a firm fixation effect without increasing the number of screws.