BACKGROUND: The incidence of congenital heart disease (CHD) has remained constant in recent years. The mortality rate is high in CHD patients admitted to the intensive care unit (ICU), but there is limited research on risk factors for in-hospital mortality. Therefore, the aim of this study was to identify risk factors for in-hospital mortality of CHD children in the ICU and develop a nomogram model to predict in-hospital mortality. METHODS: Patient demographics, comorbidities, surgical history, laboratory indicators, and in-hospital mortality were extracted from the paediatric intensive care unit (PICU) database. These patients were divided into training and validation cohorts in a 7:3 ratio. Variable selection was performed using single-factor Cox regression and stepwise Cox regression based on Akaike information criterion (AIC) in the training cohort. The selected variables were used to build a nomogram model, and calibration curves and receiver operator characteristic (ROC) curves were generated to evaluate the predictive performance of the model. Subsequently, an external validation was also carried out in the Medical Information Mart for Intensive Care III (MIMIC-III) database. RESULTS: A total of 2,231 patients were included in the analysis. Lymphocyte percentage [hazard ratio (HR): 1.097, 95% confidence interval (CI): 1.038-1.160], magnesium ion (HR: 1.002, 95% CI: 1.002-1.002), neutrophil percentage (HR: 1.111, 95% CI: 1.050-1.175), oxygen partial pressure (pO CONCLUSIONS: Neutrophil percentage in the model exhibits the strongest predictive power, followed by lymphocyte percentage and pO