OBJECTIVE: To investigate the relationship between albumin (ALB) level immediately after major abdominal surgery and postoperative acute kidney injury (AKI) in critically ill patients. METHODS: A retrospective cohort study was conducted. Patients who accepted the major abdominal surgery admitted to the department of intensive care unit (ICU) of the Peking University First Hospital from June 2017 to July 2018 were enrolled. Clinical data including the postoperative ALB level and renal function were collected. Patients were divided into postoperative AKI group and postoperative non-AKI group according to the AKI diagnosis and staging criteria of Kidney Disease: Improving Global Outcomes (KIDGO). The risk factors of perioperative AKI occurrence were analyzed, and multivariate Logistic regression analysis was performed. The receiver operator characteristic curve (ROC curve) was plotted for the ALB level to predict the occurrence of AKI and to determine the ALB cut-off value. The Kaplan-Meier survival curve of postoperative survival of patients was drawn. RESULTS: A total of 363 critically ill patients underwent major abdominal surgery, and 105 patients (28.9%) suffered from AKI. Compared with the non-AKI group, the patients in the AKI group were older (t = -2.794, P = 0.005), preoperative proportions of diabetes and chronic kidney disease were higher (χ = 4.613, χ = 5.427, both P < 0.05), the proportion of American Society of Anesthesiologists (ASA) grades and V was higher (χ = 19.444, P < 0.001), baseline serum creatinine (SCr) and preoperative brain natriuretic peptide (BNP) levels were higher (U = 2.859, U = 2.283, both P < 0.05), preoperative ALB level was lower (t = 3.226, P = 0.001), the proportion of preoperative use of contrast media was higher (χ = 7.431, P = 0.006), the proportions of emergency surgery and using vasopressor during surgery were higher (χ = 4.211, χ = 4.947, both P < 0.05), non-renal SOFA score and acute physiology and chronic health evaluation (APACHE) within 24 hours after ICU admission were higher (U = 2.233, t = 3.130, both P < 0.05), and the proportion of postoperative immediate ALB less than 32 g/L was higher (χ = 7.601, P = 0.006). ROC curve analysis showed that the cut-off value of immediate postoperative ALB for predicting postoperative AKI was 32 g/L, with the sensitivity was 86.7%, and the specificity was 28.3%. Multivariate Logistic regression analysis showed that ASA grade, use of contrast before surgery, baseline SCr and postoperative immediate serum ALB level below 32 g/L were independent risk factors for AKI [odds ratio (OR) and 95% confidence interval (95%CI) were 2.248 (1.458-3.468), 2.544 (1.332-4.857), 1.018 (1.008-1.027) and 2.685 (1.383-5.212), respectively, all P < 0.01]. Compared with the non-AKI group, the proportion of patients with AKI undergoing mechanical ventilation in ICU was higher (χ = 13.635, P < 0.001), mechanical ventilation duration, length of ICU stay, postoperative hospital stay were longer (U = 2.530, U = 5.032, U = 3.200, all P < 0.05), more postoperative complications except AKI (U = 4.799, P < 0.001), and in-hospital mortality and total hospitalization cost were higher (χ = 11.681, U = 3.537, both P < 0.001). Compared with the group with postoperative immediate serum ALB ≥ 32 g/L, the proportion of mechanical ventilation in ICU of the ALB < 32 g/L group was higher (χ = 33.365, P < 0.001), the length of ICU stay and postoperative hospital stay were longer (U = 3.246, U = 4.563, both P < 0.001), more postoperative complications except AKI (U = 3.328, P = 0.001), total hospitalization cost was higher (U = 4.127, P < 0.001). CONCLUSIONS: For critically ill patients underwent major abdominal surgery, the postoperative immediate serum ALB level below 32 g/L significantly increased the risk of AKI, which was related to the poor prognosis of the patients.