Cold legs in summer may be caused by lumbar disc herniation, physiological phenomenon, deep vein thrombosis and other reasons. 1. Lumbar disc herniation: It is mainly caused by degeneration of lumbar disc and injury of lumbar vertebrae, etc. The herniated part can compress the sciatic nerve, which leads to sensory abnormality of the innervated area, and then the phenomenon of cold legs in summer can occur. 2. Physiological phenomenon: in summer, due to often staying in air-conditioned rooms, the cold wind will stimulate the skin of the lower limbs, resulting in the slowing down of arterial blood flow and vasoconstriction, which may lead to the feeling of cold legs. 3. Deep vein thrombosis: due to the abnormal coagulation of blood in the lumen of deep vein vessels, the venous lumen is blocked, resulting in obstruction of venous return, which may cause cold legs. It is recommended that patients with cold legs in summer should go to the hospital in time and follow the doctor’s instructions for treatment to avoid delaying their condition.