A tendon sheath cyst is a benign mass containing jelly-like mucus that occurs near the joint capsule or tendon sheath and is mostly unicompartmental or multifocal. The cause is unknown. Currently, it is thought to be related to degenerative mucinous degeneration of the connective tissue on the joint capsule, ligaments and tendon sheaths due to local malnutrition or local chronic strain. It is mostly seen in young and middle-aged people, more in women than in men; most of them occur gradually or are found occasionally and grow slowly. In very few cases, the cysts can be self-absorbed, but over a long period of time. Most cases are treated non-surgically with good results, but can recur. Very few cases require surgical excision with good results. The patient was a 42-year-old male with a history of wrist pain three years ago, which had been treated twice with closure therapy, and for the past year he felt tension and swelling in his wrist. Ultrasound showed a soy-sized mass on the left wrist, and a tendon sheath cyst was considered. After incision, it was found that this soybean-sized mass, with a very enlarged tip, was located at the ulnar junction of the radial carpal flexor muscle on the deep side of the superficial finger flexor. This explains the wrist tension. If not treated surgically, carpal tunnel syndrome is bound to occur over time. Wang Xinjian, Department of Hand, Foot and Ankle Surgery, The First People’s Hospital of Ningyang County, does not recommend surgical treatment under local anesthesia and is prone to recurrence when the peeling is not clean. Good anesthesia conditions are a prerequisite for successful surgery. Postoperative pressure bandage. No stitch removal. Three weeks to fully recover.