Hepatitis B virus-related acute-on-chronic liver failure (HBV-ACLF) is a serious medical condition with a high short-term mortality rate, making accurate prognostic assessment essential for informed clinical decision-making. In this study, we aimed to develop a simple and effective prognostic model for predicting short-term mortality in patients with HBV-ACLF. Patients and Methods: To achieve our objective, we enrolled both a cross-sectional cohort (n = 291) and a retrospective cohort (n = 185) in this study. We collected laboratory and clinical data from these cohorts and performed univariate and multivariate logistic regression analyses to identify independent predictors of short-term mortality. Subsequently, we developed a novel prognostic score for HBV-ACLF, which was validated and assessed using receiver operating characteristic (ROC) curve analysis to determine its performance. Results: Our analysis revealed that the admission prealbumin (PAB) level was a robust independent predictor of 30-day mortality, with an area under the receiver operating characteristic (AUROC) of 0.760. Moreover, we developed the HIAPP score, a prognosticscore model based on PAB. The HIAPP score was significantly lower in survivors compared to non-survivors (−2.80±0.21 vs 0.97 ±0.41, P < 0.001). The HIAPP score's AUROC value was 0.899, which was found to be superior to the MELD score (AUROC = 0.795) and the CLIF-C ACLF score (AUC =0.781) and comparable to the COSSH-ACLF II score (AUC =0.825) for predicting 30-day mortality. These findings were also validated in a separate cohort, further supporting the utility of the HIAPP score as a prognostic tool for HBV-ACLF patients. Conclusion: Our study identifies the admission PAB level as a simple and valuable predictive index for 30-day mortality in HBV-ACLF patients. Furthermore, the HIAPP score, which incorporates PAB, PLT, INR, HE, and age, is an easy-to-use and pragmatic prognostic score in predicting short-term mortality.