High altitude polycythemia (HAP) refers to the excessive proliferation of red blood cells caused by people who have lived in the plateau for a long time and have lost their acclimatization to the hypoxic environment. It is the most common clinical type of chronic mountain sickness. Most people develop it at altitudes above 3200m, but a few people who are susceptible to hypoxia may develop it at altitudes below 3200m. Compared with healthy people at the same altitude, patients with HAP have significantly higher red blood cells, hemoglobin, and hematocrit, lower arterial oxygen saturation, and clinical symptoms and signs of polycythemia; pathological changes include congestion of various organs and tissues, blood stasis, and hypoxic damage.