Abstract: Background　Proximal junctional failure (PJF) is a serious complication that severely compromises outcomes in patients with degenerative spinal deformity (DSD) following long-segment spinal fusion. While bone mineral density is a key risk factor for PJF, conventional clinical assessment methods have limitations. Hounsfield unit (HU) values derived from computed tomography (CT) offer a convenient means for assessing vertebral bone quality. Objective　To investigate the predictive value of preoperative HU at the upper instrumented vertebra (UIV) for PJF, validate its independence through multivariable analysis, and subsequently establish a clinical risk stratification model. Methods　A retrospective analysis was conducted on 387 DSD patients who underwent long-segment fusion surgery. Preoperative UIV HU values were measured. Multivariable logistic regression analysis, controlling for confounders such as age and UIV level, was employed to assess the independent predictive value of UIV HU for PJF. Subsequently, receiver operating characteristic (ROC) curve analysis was performed to determine the optimal thresholds, stratifying patients into low-, medium-, and high-risk groups. PJF incidence and radiographic parameters were compared among the groups. Results　A total of 387 DSD patients were included in this study, comprising 88 males (22.7%) and 299 females (77.3%), with a mean age of 64.3 ± 5.7 years. During the follow-up period, 88 patients developed PJF, yielding an incidence rate of 22.7%. Multivariable logistic regression analysis demonstrated that for every 10-unit increase in the preoperative UIV HU value, the risk of PJF decreased by approximately 29.2% (OR=0.708, 95% CI: 0.631 - 0.792, P＜0.001). The area under the receiver operating characteristic curve (AUC) for UIV HU in predicting PJF was 0.721 (95% CI: 0.674 - 0.788). Based on the ROC analysis, thresholds of 132.3 HU (optimal cutoff) and 96.5 HU (high-specificity cutoff) were established for risk stratification. The corresponding incidences of PJF in the low- , medium- , and high-risk groups were 7.4%, 30.2%, and 45.5%, respectively (P ＜ 0.001). Radiographic analysis revealed that the high-risk group exhibited significant progression of the proximal junctional angle (PJA) during postoperative follow-up, whereas global balance parameters showed no statistically significant differences among the groups. Conclusion　Preoperative UIV HU value is an independent predictor of PJF. The dual-threshold risk stratification model constructed based on these findings can effectively identify high-risk patients, providing a quantifiable basis for preoperative assessment and individualized intervention.