Pulmonary embolism emboli are mainly thrombi. There are three factors for thrombus formation, namely blood flow stagnation, hypercoagulable state and vascular endothelial damage. In the elderly, prolonged bed rest, surgery, pregnancy, heart failure of heart disease, etc. have blood flow stagnation, which facilitates blood clotting and thrombus formation. Hypercoagulable state exists after surgery, pregnancy, malignancy, oral contraceptives and other conditions can have procoagulant substances into the blood, increased platelets and increased coagulation factors. Antithrombin III is reduced or fibrinolytic activity is diminished. Vascular endothelial injury is also an important factor. Endothelial cells release prostacyclin which has an anti-platelet aggregation effect also release tissue-type fibrinogen activator which promotes fibrinolysis once the endothelium is damaged and promotes blood clotting and thrombus formation. The emboli of pulmonary embolism mostly originate from the lower extremities, mostly deep vein thrombosis, which is reported to be 79.1% in the lower extremities, 11.5% in the pelvic veins, 8.2% in the right heart, 5.3% in the inferior vena cava and 0.4% in the others. Pulmonary infarction occurs after pulmonary embolism in more than 10% – 30%. The main reason is that the lung contains oxygen, double blood supply, both pulmonary artery supply and bronchial artery supply, and after pulmonary artery embolism, there is still bronchial artery supply, and there is anastomotic branch between them, and the anastomotic branch expands after embolism, thus reducing the occurrence of pulmonary infarction. In the study of the etiology of pulmonary embolism, in young patients under 30 years old, the underlying diseases are connective tissue disease and lower limb venous lesions, and nearly 1/3 of them have no obvious cause; in the elderly group over 60 years old, the most common cause of pulmonary embolism is cardiovascular disease, followed by malignant tumor and lower limb venous lesions; in patients aged 40-60 years old, in addition to lower limb venous lesions, surgery and malignant tumor, fracture, diabetes, kidney disease and Air travel is also a common risk factor.