The possible reasons for the tendon pain behind the ankle of the patient are as follows: 1. Achilles tendonitis: the Achilles tendon is located at the back of the ankle joint, if you often carry out strenuous sports activities or weight-bearing activities, such as hiking and climbing, going up and down stairs, it will cause the load on the Achilles tendon to increase for a long time, which will cause inflammation of the Achilles tendon, manifesting as pain in the Achilles tendon area. It may also be due to wearing inappropriate shoes, such as wearing shoes that are too small, the heel of the shoe will repeatedly rub against the Achilles tendon, and over time, there will also be inflammation of the Achilles tendon, manifested as pain in the tendon behind the ankle. 2, peroneal tendonitis: peroneal tendon is often in the tendon groove at the back of the ankle joint, around the ankle joint to reach the outside of the foot. If the peroneal tendon is rubbed, pain will also occur at the back of the outer ankle.