1, after surgery, because the surgical trauma increases the blood viscosity, and the postoperative bed braking also slows down the venous blood return rate. Especially the incidence is higher after orthopedic surgery. 2.Trauma, especially pelvic fracture and lower limb fracture, etc. 3, prolonged standing or seated immobility, such as long bus or airplane rides, especially in a long time without water blood concentration is more dangerous. 4, malignant tumor. 5, abdominal or pelvic masses compression resulting in venous return obstruction. 6.Trauma to the limb causes damage to the vein wall. 7, high blood lipids, blood viscosity increased. 8, dehydration, including insufficient intake and excessive fluid loss. 9.Oral contraceptive pills also increase the possibility of deep vein thrombosis.