Objective: To explore the evaluated value of ultrasound evaluation of renal resistance index (RRI) combined with serum cystatin C (CysC) level on acute kidney injury (AKI). Methods: The clinical data of 86 patients with AKI were retrospectively analyzed. According to the Kidney Disease Improving Global Outcomes (KDIGO) staging, they were divided into mild group (n=50) and severe group (n=36). The AKI staging was recorded, and the general data, RRI and CysC were compared between groups. ROC curve was drawn to evaluate the predictive value of CysC, RRI and their combination on patients with severe AKI. Spearman correlation analysis was used to analyze the correlation between RRI and CysC levels and AKI staging. Results: There were no statistical differences in gender and age between the two groups of patients (P>0.05). Statistically significant differences were observed in concurrent hypertension, concurrent diabetes mellitus and etiology in severe group compared with those in mild group (P<0.05). The RRI and CysC in severe group were higher than those in mild group, with statistical differences (P<0.05). ROC curve analysis showed that the AUCs of CysC, RRI and combined prediction in predicting severe AKI were 0.807, 0.773 and 0.903. The optimal cut-off value of CysC was 1.96 μmol/L, with a sensitivity of 68.00% and a specificity of 86.11%. The optimal cut-off value of RRI was 0.68, and the sensitivity and specificity were 58.00% and 86.11%. The sensitivity and specificity of combined prediction were 100.00% and 74.00%, respectively. Spearman correlation analysis showed that RRI and CysC were positively correlated with AKI staging (P<0.05). Conclusion: Most patients with AKI are in the early stage, and hypertension and diabetes mellitus are related to the disease severity. Ultrasound evaluation of RRI combined with serum CysC alone and in combination in predicting severe AKI has value. Combined prediction can enhance the predictive sensitivity, and the levels of the two are positively correlated with AKI staging.