A swollen but not painful ankle can be caused by several reasons: First, vascular disease: varicose veins or venous thrombosis in the lower extremities can cause swelling of the ankle, but no painful symptoms. In this case, ultrasound and angiography of blood vessels should be performed to diagnose and treat. Second, systemic diseases: kidney diseases, heart diseases and hypoproteinemia can lead to ankle swelling. In this case, examination of related organs and blood tests should be performed for diagnosis and early treatment. Third, old trauma: A part of patients with old trauma can also lead to swelling of the ankle without obvious pain, in which case diagnosis and treatment should be made according to the patient’s medical history and physical examination. Fourth, swelling: tendon sheath cysts around the ankle can also appear as localized mass swelling, and if in the early stage, it can also not appear painful, then the diagnosis and treatment should be made through relevant ultrasound and MRI.