Objective To evaluate the effect of fluid overload on the prognosis of children receiving continuous renal replacement therapy (CRRT) after cardiac surgery. Methods We collected 48 children receiving CRRT after cardiac operation from June 2005 to June 2010. Fluid overload in the period from admission to ICU to initiation of CRRT was expressed as: [total fluid intake (L) – total fluid output (L)] / [body weight at admission to ICU (kg)] x 100%. The patients were divided into two groups: fluid overload ＜10% group and fluid overload ≥10% group. Their clinical manifestations and outcomes were recorded and analyzed. Results In the 48 cases, 26 cases (54.2%) were found to have fluid overload <10%, and 22 cases (45.8%) to have fluid overload ≥10%. The mortality rate was significantly higher in children with fluid overload ≥10% (14/22; 63.6%) than in those with fluid overload ＜10% (9/26; 34.6%). Multivariate logistic regression analysis demonstrated that the degree of fluid overload was significantly correlated with the mortality rate (OR=1.05; 95%CI=1.02-1.08). Conclusions The prognosis of the children receiving CRRT after cardiac surgery correlates to the degree of fluid overload. Appropriate fluid management before CRRT may improve the prognosis.