The main causes of pulmonary thromboembolism are venous thrombosis of the lower extremities and the formation of pelvic vein thrombosis. The main causes of venous thrombosis are stagnation of venous blood, damage to blood vessels and changes in blood rheology, increased blood viscosity. Common risk factors for the formation of venous thrombosis of the lower extremities are prolonged bed rest of the patient, trauma, tumor, surgery, oral contraceptives, blood clotting factors, etc. Some patients with impaired laboratory indicators, including some hereditary diseases, some systemic diseases such as immune deficiency, lupus erythematosus, rheumatoid, etc., are prone to hemodynamic changes causing the formation of lower extremity venous thrombosis, which leads to pulmonary artery thromboembolism.