Objective To evaluate the early diagnosis value of serum cystatin C in the pediatric intensive care unit (PICU) children with septic acute kidney injury.Methods A total of 196 children in PICU confirmed with sepsis in Hunan Province Children's Hospital was enrolled in this study.Patients were divided into acute kidney injury (AKI) and non-AKI group according to whether accompanied with acute kidney injury.The serum cystatin C and serum creatinine were collected in 2 h,48 h,and 96 h after admission,and the clinical data were collected.The serum Cys C was drawn in receiver operating characteristic (ROC) curve.The sensitivity and specificity of Cys C were evaluated in diagnosis of septic AKI.Results The incidence of septic AKI was 35.20％,higher Cys C levels were risk factors for the onset of AKI,and OR was 26.218 (95％ CI:6.235 ～ 110.232).In AKI group,the serum Cys C level in 2 hours after admission was (2.05 ± 0.90)mg/L,which was higher than 48 hours (1.72 ± 0.72)mg/L and 96 hours (1.62 ±0.95) mg/L(Z =2.169,P =0.030; Z =2.789,P =0.005).In the septic AKI group,cystatin C and creatinine were positive correlation (r =0.582,P =0.000).The area under the ROC curve at 2hours after admission for serum cystatin C in diagnosis of AKI was 0.831.A cutoff point of 1.325 mg/L for 2 hours after admission was identified for cystatin C in the diagnosis of septic AKI,with a sensitivity of 87.1％and specificity of 78.9％.Conclusions Higher level of Cys C was risk factor for the onset of AKI.Compare to creatinine,cystatin C was earlier increased in children with septic AKI and positively correlated with it.It might be a biomarker for early diagnosis of septic AKI in PICU critical ill children.