Background Acute kidney injury (AKI) is widespread in the intensive care unit (ICU) and affects patient prognosis. According to Kidney Disease: Improving Global Outcomes (KDIGO) guidelines, the absolute and relative increases of serum creatinine (Scr) are classified into the same stage. Whether the prognosis of the two types of patients is similar in the ICU remains unclear. Methods According to the absolute and relative increase of Scr, AKI stage 1 and stage 3 patients were divided into stage 1a and 1b, stage 3a and 3b groups, respectively. Their demographics, laboratory results, clinical characteristics, and outcomes were analyzed retrospectively. Results Of the 345 eligible cases, we analyzed stage 1 because stage 3a group had only one patient. Using 53 or 61.88 mu mol/L as the reference Scr (Scr(ref)), no significant differences were observed in ICU mortality (P-53 =0.076, P-61.88 =0.070) or renal replacement therapy (RRT) ratio, (P-53 =0.356, P-61.88 =0.471) between stage 1a and 1b, but stage 1b had longer ICU length of stay (LOS) than stage 1a (P-53 <0.001, P-61.88 =0.032). In the Kaplan-Meier survival analysis, no differences were observed in ICU mortality between stage 1a and 1b (P-53 =0.378, P-61.88 =0.255). In a multivariate analysis, respiratory failure [HR = 4.462 (95% CI 1.144-17.401), p = 0.031] and vasoactive drug therapy [HR = 4.023 (95% CI 1.584-10.216), p = 0.003] were found to be independently associated with increased risk of death. Conclusion ICU LOS benefit was more prominent in KDIGO(SCr) AKI stage 1a patients than in stage 1 b. Further prospective studies with a larger sample size are necessary to confirm the effectiveness of reclassification.