Objective The study aimed to construct one severity of illness scoring system specially for children with complex congenital heart diseases and analyzed the applicability of this new score,which will be helpful to assist the medical workers in evaluating children's condition.Methods A total of 1284children patients with complex congenital heart diseases in Shanghai Children's Medical Center Affiliated to School of Medicine of Shanghai Jiaotong University.The general information and clinical data were collected.Logistic regression,method of percentiles and expert opinions were used to construct the new severity of illness scoring system.Area under ROC curve,Hosmer-Lemeshow,goodness of fit test were used to evaluate the performance and applicability of the new score.Results A total of 1284 children with complex congenital heart diseases split into construction group (1138 cases) and verification group (146 cases).The new score included seven indexes (systolic blood pressure,blood oxygen saturation,lactic acid,potassium,potential of hydrogen,glucose,positive inotropic drug score).The range of the score was 0-57 and the cut-off point was 10.The discriminatory power was sufficient (area under the curve ＞ 0.75),and the calibration capability of the score was tolerable (P ＞ 0.05).Conclusions The applicability of the new score is well and it is easy to use,which will not increase the workload for the medical workers while assisting them in the clinical environment.