A normal atrium is the presence of an atrial septum between the left atrium and the right atrium with no blood flow. An atrial septal defect refers to the presence of an abnormal foramen ovale defect in the septum of both atria, which can cause the left and right atria to communicate directly. Atrial septal defect is one of the most common congenital heart diseases. The formation of an atrial septal defect is due to an abnormality in the primitive atrial septum during fetal development, resorption and fusion, resulting in a hole-like defect of variable location, shape and size in the atrial septum, where blood flows directly between the left and right atria and abnormal blood flow within the heart occurs. Because the pressure in the left atrium is greater than that in the right atrium, a left-to-right shunt exists in the atrial septal defect at the level of the atria. The size of the shunt flow depends mainly on the size of the septal defect and the pressure step difference between the left and right atria. The early onset and severity of clinical symptoms depend on the size of the defect. In infancy, the left-to-right shunt through the atrial septal defect is small, so it does not affect cardiac function much, nor does it affect strenuous activity. However, as we grow older, the shunt flow increases further and clinical symptoms become more pronounced. Patients with atrial septal defect may show symptoms such as easy fatigue after activity, shortness of breath, chest tightness, growth retardation, feeding difficulties, etc. in their early years, and some patients may repeatedly cause pneumonia. When the disease progresses further and the pressure in the right atrium is higher than that in the left atrium, blood flow from the right atrium through the atrial septal defect produces a right-to-left shunt, which is the manifestation of Eisenmenger syndrome. The patient’s symptoms gradually worsen with hemoptysis, cyanosis, and right heart failure. For large atrial septal defects, the presence of a large left-to-right shunt with pulmonary hypertension should be operated as early as possible. Some small atrial septal defects with small shunt volumes and relatively small defect diameters can be closely monitored.