Objective：To investigate the association between first-day serum albumin（ALB）levels upon intensive care unit（ICU） admission and in-hospital mortality in patients with acute pancreatitis（AP），and to explore its potential non-linear nature. Methods： This retrospective study analyzed data from 728 adult AP patients admitted to the ICU for the first time from2008 to 2019，sourced from the MIMIC - IV database. First -day ICU ALB levels and relevant clinical data were extracted. Multivariate logistic regression models were used to assess the linear relationshipbetween ALB and in - hospital mortality. Generalized additive models（GAM）and two - piecewise linear regression models were employed to explore non -linear relationships and identify inflection points. Results：After adjusting for multiple confounders，each 1 g/L increase in first - day ICU ALB was associated with a 27% reduction in in - hospital mortality risk（OR=0.73，95% CI：0.59-0.92，P=0.007）. GAM analysis further revealed a significant non-linear relationship between ALB and in-hospital mortality，with an inflection point at 28 g/L. When ALB was≤28 g/L，each 1 g/L increase in ALBwas associated with a 56% significant reduction in mortality risk（OR=0.44，95%CI：0.26-0.73，P=0.001）；however，no statistical significance was observed when ALB ＞ 28 g/L. Conclusion：First-day ICU serum ALB levels in patients with AP demonstrated a significant non-linear negative association with in - hospital mortality. An ALB level of 28 g/L was identified as an important clinical inflection point for distinguishing high-risk patients. Early monitoring and attention to ALB levels are crucial for risk stratification and clinical decisionmaking in AP patients