Abstract: Objective To assess the radiographic parameters of S1 and S2 sacral alar iliac (SAI) screws in Chinese adults population and compare the parameters of S1-AI and S2-AI techniques. Methods Three dimensional computed tomography (3DCT) reconstructions of 60 randomly selected Chinese skeletally mature patients with normal pelvis in our hospital from April to October in 2015 were analyzed.Trajectories of S1AI and S2AI techniques with maximal length and width were obtained by manipulating and rotating the 3D pelvis.The length and angulation parameters were evaluated and compared. Results Based on the ideal trajectory, the parameters of S1AI and S2AI technique were shown as follows: the S1AI trajectories were more caudal than S2AI trajectories with average sagittal angulation increasing 26° in male and 24° in female.The sagittal angulation was (57.25±4.37)° (S1) vs (31.02±7.43)° (S2), P< 0.05 in male and (58.61±5.11)° (S1) vs (34.16±6.02)° (S2), P< 0.05 in female.The lateral angulation of S1AI trajectories was 2° less than that of S2AI trajectories on average, which was (38.14±3.10)° (S1) vs (40.25±2.84)° (S2), P< 0.05 in male and (37.24±2.96)°(S1) vs (39.25±2.64)° (S2), P< 0.05 in female.The maximal length of trajectories was (119.46±3.77) mm (S1) vs (120.04±6.52) mm (S2), P=0.551 in male, and (108.21±5.24) mm (S1) vs (109.47±4.63) mm (S2), P=0.434 in female.The length of sacroiliac screws in male was (43.12±4.63) mm (S1) vs (35.75±5.03) mm (S2), P< 0.05, and (39.73±5.85) mm (S1) vs (32.16±5.28) mm (S2), P< 0.05 in female, and the width of it was (22.05±4.91) mm (S1) vs (21.49±3.22) mm (S2), P=0.672 in male and (16.93±2.12) mm (S1) vs (15.35±2.71) mm (S2), P=0.366 in female.The distance to cortical bone was (9.42±2.41) mm (S1) vs (9.01±2.33) mm (S2), P=0.352 in male and (5.92±1.48) mm (S1) vs (6.34±1.26) mm (S2), P=0.837 in female.There were significant differences in the distance from screw entrance point to skin, median line and spina iliaca posterior superior between S1AI and S2AI technique. Conclusion The application of S1AI and S2AI screws into the sacrum and ilium of Chinese adult population is feasible along with the angle and length of ideal trajectory.The S1AI trajectoriesare more caudal than S2AI trajectories with average sagittal angulation increasing 26° in male and 24° in female, and the lateral angulation of S1AI trajectories is 2° less than that of S2AI trajectories on average.The maximal lengthes of trajectories are almost the same, while the distance from screw entrance point to skin and spina iliaca posterior superior of S1AI is much deeper, and the distance from screw entrance point to median line is longer.Differences between S1AI and S2AI ideal trajectories should be noticed in operation.