The most common causes of thigh purple when standing and lying down will disappear are lower extremity venous thrombosis, vasculitis, varicose veins of the lower extremities. 1. Lower extremity venous thrombosis: Various reasons cause poor venous return in the lower extremities, which is easy to form venous thrombosis in the lower extremities, and the patient may show bruising and purpling of the skin when standing, and it may disappear after lying down and the pressure on the blood vessels is reduced. 2. Vasculitis: this is a kind of arterial, venous and capillary lesions, usually immune inflammatory changes in the walls of the blood vessels and their surroundings, which in turn cause narrowing of the blood vessels, circulatory blockage and other manifestations, and the patients may show bruises and petechiae when standing, which may disappear when the pressure on the blood vessels is reduced after lying down. 3. Varicose veins of the lower limbs: In patients with varicose veins of the lower limbs, the varicose veins are more obvious and full when they stand up, and the pressure of blood on the surrounding walls will decrease after lying down, and then they may disappear. There are many reasons why thighs become purple when standing and disappear when lying down. If the symptoms don’t get better after resting, it is recommended to go to the hospital in time to find out the cause of the disease and then carry out targeted treatment.