The swelling of lower limbs is often neglected and mistakenly thought to be caused by exertion and other reasons, thinking that it will turn out well after resting, thus missing the best time for treatment. Therefore, sudden swelling of the lower limbs, especially unilateral swelling of the lower limbs, must not be taken lightly, because it is likely to suffer from deep vein thrombosis of the lower limbs. So what is lower extremity deep vein thrombosis? In layman’s terms, it is a clotting of blood that blocks the veins of the lower limbs. As we know, the blood circulation of human body is composed of two parts: artery and vein, the venous blood flows into the heart, and then the arterial blood flows out from the heart. The blood circulation is less than one second, and once the clot is dislodged, it quickly flows back to the heart and out of the heart, and flows to the lungs for pulmonary embolism; to the brain for cerebral infarction; to the coronary arteries for myocardial infarction, all of which can be fatal, so we must raise awareness. So what exactly can cause lower extremity deep vein thrombosis? There are three main causes of lower extremity deep vein thrombosis: ① venous intima injury ② slow blood flow ③ blood hypercoagulation state. Venous endothelium has good anticoagulation and inhibit platelet adhesion and aggregation function, and intact venous endothelium is the prerequisite to prevent deep vein thrombosis, while the common causes of venous injury are: trauma, surgery, trauma, electric shock, infection damage, etc. Moreover, lower limb deep vein thrombosis often occurs under various braking conditions, such as after various surgeries, serious illness in bed, fracture fixation, pregnancy, prolonged sedentary (such as boat, car, fishing, playing cards, etc.). Therefore, once the swelling of the lower limbs occurs and there are reasons for appeal, the occurrence of lower limb deep vein thrombosis is highly suspected. It is better to treat lower limb DVT early than late, the more timely the treatment, the better the effect, the best treatment time is within 6 hours of the onset, so once the lower limb swelling is found, seek medical attention as soon as possible, do not wait and see.