Aortopulmonary septal defect (APSD), also known as aortopulmonary window (APW), is a rare congenital cardiovascular malformation, with a history of nearly 200 years since it was first reported by Elliotson [1] in 1830. The incidence of congenital cardiovascular anomalies accounts for 0.1% to 0.3% of congenital cardiovascular anomalies, with some reports of 0.2-0.6% [2] and even up to 1% [3], probably due to small sample size or racial and geographical differences. The decrease in postnatal pulmonary vascular resistance exacerbates the left-to-right shunt at the level of the great arteries in patients with APSD, resulting in heart failure and recurrent lower respiratory tract infections, which, without surgical treatment, progresses to pulmonary hypertension and irreversible changes in pulmonary vascular structure that can lead to loss of surgical opportunities. Approximately 40% of infants die in the first year, and most survivors die of heart failure in childhood. The prognosis is good if the diagnosis is made before irreversible changes in the pulmonary vasculature occur and surgery is performed as early as possible [4]. Therefore, early diagnosis is crucial to the prognosis of patients, and this paper reviews the diagnosis of main pulmonary artery septal defect (APSD). Occurrence, staging and combined malformations of APSD The cause of APSD is not clear, with no significant difference in the incidence between men and women, and no genetic or environmental-related causative factors have been identified. During the 5-8 weeks of embryonic development, the right and left bulbous cristae formed by the arterial trunk and the subendocardial tissue of the cardiac bulb fuse in the midline, and the main pulmonary artery septum forms in a spiral walk. The main pulmonary artery septum is fully formed when the 6th pair of arterial arches distal to the main pulmonary artery forms the left and right pulmonary arteries. If the cristae fuse or the 6th aortic arch migrates abnormally, it creates a traffic between the ascending aorta and the main pulmonary artery or the right pulmonary artery originates directly from the ascending aorta, i.e., an aortopulmonary septal defect [5], the size of which varies from a few millimeters to several centimeters. There are various methods of typing APSD, and the most common one abroad is the Mori et al classification [6], type I (proximal type): septal defect between the posterior wall of the ascending aorta and the proximal part of the pulmonary artery trunk, with little or no septal tissue below it, and the defect can be continued upward by the semilunar valve, accounting for about 70% of cases; type II (distal type): septal defect between the posterior wall of the ascending aorta and the beginning of the right pulmonary artery, with the septum below it Type II (distal): septal defect between the posterior wall of the ascending aorta and the beginning of the right pulmonary artery, with the septum below separating the 2 groups of semilunar valves and the septum above it having less tissue or absence, accounting for about 25% of cases; Type III (mixed): complete defect of the main pulmonary septum, accounting for about 5% of cases. The Richardson typing is also commonly used in China, with the first two types being the same as the Mori et al classification, and the right pulmonary artery anomaly originating in the ascending aorta as type III [7]. Based on recent years, when some APSDs can be closed with a blocker, Jacobs et al improved on the Mori classification by adding type IV: a transitional defect with septal tissue all around the defect, which may be suitable for blocking. This is also the classification recommended by the American College of Thoracic Surgeons Committee on Precardiac Disease (Figure 1) [8]. However, Chen, Ming-Ren et al. reported a case of APSD in 2006, which was a traffic between the apex of the main pulmonary artery and the aortic arch, and this type of defect has not been included in the staging criteria [9].