When atrial septal defect exists, blood flows from left to right bypass, right heart blood volume increases, right atrial and right ventricular enlargement occurs, ventricular wall thickens, pulmonary arteries dilate to different degrees, pulmonary circulation blood volume increases, and pulmonary artery pressure rises. With the development of the disease, intimal hyperplasia, thickening of the middle layer and narrowing of the lumen occur in the wall of small pulmonary arteries, thus the pulmonary vascular resistance increases, pulmonary hypertension changes from dynamic to resistance, the pressure in the right atrium and right ventricle increases, the pressure in the right atrium exceeds that in the left atrium in the late stage of the disease, a right-to-left shunt occurs, and clinical purple crucible and heart failure appear.