Abstract: Objective To explore the role of pelvic incidence minus lumbar lordosis (PI-LL) in maintaining sagittal balance after osteotomy in patients with ankylosing spondylitis-related kyphotic deformity. Methods A total of 105 patients with ankylosing spondylitis-related kyphotic deformity who were admitted to Nanjing University Medical School Affiliated Drum Tower Hospital from August 2019 to August 2021 were enrolled. Their clinical data were collected for retrospective analysis. All patients underwent osteotomy. Their sagittal parameters of the spine and pelvis were recorded before surgery, at postoperative 4 weeks, and at final follow-up postoperative 24 months. Pearson correlation analysis was used to assess the relationship between postoperative PI-LL and sagittal parameters of the spine and pelvis at final follow-up. Receiver operating characteristic (ROC) curves were plotted to determine the optimal cutoff value for PI-LL postoperatively. Results The sagittal vertical axis(SVA) of patients at postoperative 4 weeks and final follow-up were (9.2±4.4) cm and (8.6±5.0) cm, respectively, and the lumbar lordosis (LL) were (-33.8±15.8)° and (-32.0±16.8)°, which were significantly improved compared with those before surgery (P<0.001). The pelvic tilt (PT) and sacral slope (SS) at postoperative 4 weeks were (28.7±9.7)° and (20.7±11.0)°, and at final follow-up were (31.4±11.3)° and (15.7±11.4)°, which were significantly improved compared with those before surgery (P<0.001). The PI-LL at postoperative 4 weeks and final follow-up were (14.8±4.5)° and (15.9±5.2)°, respectively, showing significant improvement compared with preoperative levels (P<0.001). Pearson correlation analysis indicated a significant positive correlation between postoperative PI-LL and SVA (r=0.627, P=0.001) and PT (r=0.688, P<0.001) at final follow-up. ROC curve analysis showed that the optimal postoperative PI-LL was ≤10.7°. Conclusions Maintaining a PI-LL of ≤10.7°can effectively preserve sagittal balance in patients with ankylosing spondylitis-related kyphotic deformity after osteotomy.