There are many reasons for unilateral ankle swelling and pain, the common ones are ankle sprain, osteoarthritis, rheumatoid arthritis and so on. 1. Ankle sprain: unilateral ankle joint exceeds its normal mobility during walking or exercise, causing damage to the soft tissues around the joint, such as ligaments and muscles, resulting in ankle swelling and pain. 2. Osteoarthritis: damage to joint cartilage is the main cause, involving the entire joint tissue, and eventually joint cartilage degeneration, fibrosis, fracture, etc. Osteoarthritis occurring in the ankle joint can appear ankle joint pain, stiffness, pain, and pain can be seen in the joints when standing still or when changing the position in the morning, and the pain is gradually alleviated after activities. The disease can be triggered by cold and exertion. 3. Rheumatoid arthritis: mainly characterized by erosive arthritis, it can occur in various joints, such as the ankle joints, with morning stiffness, swelling and pain in the joints in the early stage of the disease, and finally joint deformity can occur. Unilateral ankle swelling and pain may have other causes such as gout, etc. It is recommended to go to the hospital in a timely manner, improve the examination to clarify the cause of the disease and standardize the treatment.