Old thrombus in the veins of the lower limbs usually does not need special treatment, and those who are at risk of thrombosis can take medication to prevent thrombosis. Because thrombosis in the acute period is easy to dislodge and lead to the occurrence of pulmonary embolism, and with the prolongation of time, the lower extremity thrombus gradually stabilized, mechanization, and the wall of the blood vessel appears to be adherent, but also will form a part of the regenerative blood vessels, called collateral circulation. After the establishment of collateral circulation, it can compensate for the original embolized blood vessels, and the original blood clots in the blood vessels gradually mechanize with the extension of time, and no longer dislodge to cause pulmonary embolism. Therefore, no special treatment is needed for old thrombus. However, if the patient has a high risk of thrombosis, for example, if the patient needs to stay in bed for a long time or if there are other factors that promote thrombosis, then it is necessary to follow the doctor’s instructions to take low molecular heparin or apply rivaroxaban to prevent new thrombosis.