Abstract: 　Objectives　To investigate the association of fluid overload (FO) with the development and mortality of acute kidney injury (AKI) and to evaluate the predictive value of FO in mortality of critically ill children. Method　A prospective study was conducted among critically ill children who were admitted to the children's intensive care unit (PICU). FO levels were assessed during the course of the disease and PRISM Ⅲ scores were evaluated within 24 hours of admission. Binary logistic regression analyses were conducted to evaluate the association of FO with the development and mortality of AKI after adjusting for confounding factors. The area under the receiver operating characteristic curve (AUC) was calculated to assess the predictive value of FO for mortality. Results　In 362 children included, there were 26 children (7.18%) having average FO≥5%, and AKI in 24 children (6.63%) and 18 children (5.0%) died. The mean FO (OR=1.26, 95%CI: 1.10~1.43, P=0.001) and the maximum FO (OR=1.12, 95%CI: 1.02~1.23, P=0.018) were significantly correlated with the development of AKI in critically ill children within 7 days of admission to PICU. However, after adjusting for age and PRISM Ⅲ, both factors had no association with AKI (all P>0.05). After adjusting for the potential confounders such as AKI and the severity of disease, the average FO was significantly associated with mortality (AOR=1.34, 95%CI: 1.12~1.60, P=0.002). The AUC of mean FO that predicted mortality risk was 0.801 (P<0.001). Conclusion　Fluid overload is associated with the development and the prognosis of AKI in critically ill children, and has important predictive value for mortality.