Pulmonary arterial hypertension (PHA) is a syndrome in which the lesion involves the endothelium, myocardium, and epicardium of the pulmonary arteries, resulting in increased pulmonary vascular resistance due to restriction of pulmonary blood flow and ultimately right heart failure. The syndrome is characterized by increased pulmonary artery pressure and normal pulmonary venous pressure, and the need for normal cross-pulmonary pressure differential. In 1973, the first primary PAH conference held by the World Health Organization (WHO) divided pulmonary hypertension into two major categories: primary PAH and secondary PAH. 1988 French Evian conference divided PH into five major categories according to the pathology, pathophysiological features and treatment of pulmonary circulatory hypertension (PH), and the Evian diagnostic classification was revised at the 2003 Venice conference. The fourth PH meeting held in DaNa Point, USA in 2008 reached a consensus to update the diagnostic classification of PH after discussion. The latest classification differs from the Venice classification in the following ways: 1) The first major category of PAH: (1) The name of familial PAH diagnosis was eliminated in favor of hereditary PAH because some patients with disseminated idiopathic PAH carry specific genetic mutations despite having no family history, while some familial PAH are not found to have specific genetic mutations. The new classification does not require genetic testing for hereditary PAH because it is not helpful for treatment. (2) Clinical classification and anatomic-pathophysiologic classification of PAH associated with congenital cardiac vascular disease (precordial disease). PAH persisted after cardiac malformation repair, which improved significantly postoperatively but was significantly worse again months or even years later, and there was no significant postoperative residual fistula. (3) Greater variation in PAH due to related factors, classifying schistosomiasis and chronic hemolytic anemia (e.g., sickle cell disease, thalassemia, etc.) in this category. 2. Since pulmonary vein occlusive disease and capillary hemangioma have both commonalities and significant differences with idiopathic PAH, it is difficult to completely separate them from PAH. 3. The second major category (left heart disease-related) and the third major category (respiratory disease or hypoxia-related) were not revised. 4. The fourth major category (chronic thromboembolic PH, CTEPH) is no longer divided into proximal and distal pulmonary artery embolism because there is no accurate criterion to distinguish between the two. 5. The fifth major category (PH due to unknown mechanism or multiple factors) classifies diseases with unknown pathogenesis into this category, including hematologic diseases other than hemolytic anemia, systemic diseases, metabolic diseases and other rare diseases.