Background <p>Chronic kidney disease (CKD) is a common complication among patients in the intensive care unit (ICU), however, the impact of pre-existing CKD on patient who were admitted to the ICU and diagnosed with acute kidney injury (AKI) remains controversial.</p> Methods <p>Critically ill patients (18–90 years old) with AKI were enrolled as study participants from the Medical Information Mart for Intensive Care-III database. Patients with repeated ICU admissions and a length of stay less than 48 h were excluded. In-ICU mortality was considered the main endpoint, and multivariable Cox regression analyses were performed. Moreover, propensity score matching (PSM) was employed to adjust potential interference factors, and the three-year survival rate was analyzed using the Kaplan–Meier method.</p> Results <p>This study included 20440 patients, divided into the pure AKI group (18441 patients) and the acute-on-chronic kidney disease (ACKD) group (1999 patients). Multivariable Cox regression analyses revealed lower in-ICU mortality in ACKD group than pure AKI group (7.0% versus 7.7%, hazard ratio (HR) 0.83, 95% confidence interval (CI) 0.69–0.99, P = 0.047), as well as after PSM (7.0% versus 9.9%, HR 0.79, 95% CI 0.63–0.99, P = 0.049). Statistically significant differences persisted when patients were stratified by AKI stages. The results demonstrated that preexisting CKD was associated with reduced in-ICU mortality among AKI stage 3 patients, regardless of the classification criteria used. However, the Kaplan–Meier method indicated a lower three-year survival in the ACKD group (P = 0.017).</p> Conclusion <p>Among AKI patients in ICU, the ACKD group had lower ICU mortality compared to pure AKI group but experienced worse long-term survival.</p>