There is a pimple on the surface of the foot for the following common reasons: first, tendon sheath cyst, which is caused by the protrusion of the joint cavity, generally do not worry too much, need to reduce activities, can be localized gently massage, if necessary, can be punctured to draw fluid, sometimes need to perform surgical excision treatment. Second, local skin infection or sebaceous cyst formation, which needs to go to the hospital to perform physical examination or ultrasound to clarify, if necessary, surgery to remove treatment. Third, bony hyperplasia caused by the need to go to the hospital to take foot X-rays or foot CT to clarify. Fourthly, it may be caused by mosquito bites, and local anthritis, mosquito repellent or sodium bicarbonate solution can be applied to relieve the symptoms. Fifth, there are other causes, such as thrombosis or enlarged lymph nodes. In short, if the surface of the foot continues to have a swelling does not subside, you need to go to the hospital in order to clarify the cause.