The possible mechanisms of preoperative combined malnutrition in children with congenital heart disease are: in left-to-right shunt congenital heart disease, pulmonary hypertension, pneumonia and respiratory insufficiency due to congestion in the pulmonary circulation, causing feeding difficulties; cardiac insufficiency and venous system stasis leading to intestinal dysfunction and impaired nutrient digestion and absorption; and in right-to-left shunt congenital heart disease, due to peripheral circulation hypoxia and acidosis, affecting energy metabolism in the body.