A swollen lump on the back of your foot that makes it painful to walk may be due to a tendon sheath cyst. Tendon sheath cysts are soft tissue disorders that usually appear as a smooth, soft, benign lump that protrudes from the skin. The inside of the cyst is filled with a viscous fluid or jelly-like substance that is soft to the touch and can be pressed. The site of onset is commonly the dorsal aspect of the wrist and the dorsal aspect of the foot. The etiology is unknown and may be related to degeneration of adjacent tissues and long-term irritation at the site of injury. Tendon sheath cyst is a common benign swelling, if the local symptoms are not serious, temporary observation can be. Mild pain can be relieved by oral non-steroidal anti-inflammatory drugs as prescribed by the doctor. General treatment can take puncture, suction, injection of steroids, but it is easy to recur. If the cyst continues to increase in size, or symptoms such as pain and numbness worsen, surgical treatment can be performed. There may also be other reasons, it is recommended to go to the hospital in time, improve the examination to clarify the cause of the disease, and then give targeted treatment or treatment.