Objectives: To observe the incidence of cerebrospinal fluid leakage in the surgery of thoracic ossification of ligamentum flavum by posterior laminectomy, and discuss the related risk factors, aiming to provide evidence for clinical prevention. Methods：The study design is a retrospective cohort study. The clinical data of patients diagnosed with thoracic ossification of ligamentum flavum and treated with posterior laminectomy in our hospital from August 2015 to August 2021 were analyzed. The patients were divided into the CSFL group and the non-CSFL group according to whether cerebrospinal fluid leakage occurred. The differences of baseline data, imaging data and surgical data between the two groups were retrospectively analyzed by single factor and binary multivariate logic. Results：108 patients were included in this study. 28 were complicated with cerebrospinal fluid leakage (25.93%). Univariate analysis showed that there were statistically significant differences between the two groups in operative segments, operative time, surgical instruments, blood loss, length of hospital stay, residual rate of spinal canal area, residual rate of axial median diameter and sagittal diameter (P < 0.05). Multivariate logistic regression analysis showed that the risk of CSFL increased 3.27 times with each additional surgical level. The risk of CSFL increased 0.97 times for each 10% reduction in the residual rate of spinal canal area. Conclusion： More surgical levels and lower residual rate of spinal canal area are associated with higher risk of CSFL. The corresponding measures should be taken to reduce the occurrence of CSFL.