1, vascular damage: surgery, trauma, fracture, chemical drugs and some other factors can directly lead to vascular wall damage, when the venous injury subendothelial layer and collagen exposed, so that the venous wall charge change, easy to cause platelet adhesion; trauma endothelial cell function damage, can release bioactive substances, start the endogenous coagulation system, easy to form thrombus; so platelets due to the venous wall charge change or due to Endothelial cell damage when the coagulation system is activated and adhesion, aggregation of thrombus formation. 2.Slow blood flow: prolonged bed rest, physiological reaction during surgery, postoperative limb braking, sedentary state or vascular compression stenosis can cause slow blood flow in the limb. The slow blood flow leads to the formation of vortex in the valve sinus; the local hypoxia of the valve causes the expression of leukocyte adhesion factor and leukocyte adhesion contributes to thrombus formation. 3, blood hypercoagulation: pregnancy, postpartum, long-term use of contraceptives, tumor tissue lysis products, large burns and other factors can make the blood in a hypercoagulable state. At this time, the platelet count increases, the content of coagulation factors increases and the activity of anti-coagulation factors decreases and the thrombus is formed. 4, thrombus morphology: the typical thrombus includes three parts: head, neck and tail. The head is white thrombus (including fibrin, lamellar platelets and leukocytes, and very few red blood cells); the neck is mixed thrombus (a mixture of white and red thrombus); the tail is red thrombus (platelets and leukocytes are scattered in a reticulated block of red blood cells and fibrin). 5, thrombus regression: thrombus can grow and spread to the distal and proximal ends. Later, under the action of fibrinogen lysis enzyme, the thrombus can be dissolved and dissipated, sometimes the small cleaved embolus will enter the lung with the blood and cause pulmonary embolism. When the thrombus formation cannot be completely dissolved and dissipated, a fissure can be formed in the vein called incomplete recanalization.