The spontaneous rupture of the thumb flexor tendon is a rare case, and a recent case admitted to our department is reported as follows: The patient, Li Jinwa, male, 64 years old, farmer, was admitted to the hospital with the chief complaint of “left thumb flexion dysfunction for more than one month”, and more than one month ago, the patient found difficulty in tying his shoelace in the morning, and then found loss of interphalangeal joint flexion function of the left thumb. He had a navicular fracture of the left wrist nearly 40 years ago and had not received any formal treatment. On examination, the left wrist joint was slightly enlarged and deformed, with a range of motion from 0 to 40 degrees of flexion and extension, limited dorsal extension, positive Tinel’s sign of the left median nerve, significant radial pain in the left wrist joint, loss of flexion of the left interphalangeal joint of the thumb, and good passive flexion and extension. Spontaneous rupture of the tendon. The procedure was as follows: a curved incision was made in the carpal palm to protect the median nerve and its return branch, and the carpal tunnel was explored. /The tendon fibers were destroyed. The carpal tunnel segment of the median nerve was slightly thickened. The thumb flexion function was reconstructed by occluding the sharp edge of the fracture, suturing the soft tissue of the posterior wall of the carpal tunnel, performing epicranial release of the median nerve, and transposing the superficial flexor tendon of the ring finger. The total activemovement (TAM) of the joint was used as a method to assess the function of the tendon, and the results showed that the flexion function of the left thumb was completely restored, and the treatment effect was good. Discussion: Spontaneous rupture of the thumb flexor tendon is rarely reported. Intraoperatively, it was found that a navicular fracture and puncture of the soft tissue of the posterior wall of the carpal tunnel to invade the carpal tunnel were the key factors of tendon rupture. The total non-healing rate of navicular fracture was about 10% [1], and the fractured end was sclerotic and sharp. In addition to the rupture of the thumb flexor tendon, the fibers of the deep flexor tendon of the index finger were severely worn, and it can be speculated that the spontaneous rupture of the deep flexor tendon of the index finger may also be complicated. The patient’s perilunate dislocation increased the friction between the tendon and the posterior wall of the carpal tunnel, which accelerated the process of tendon rupture. In this case, the patient had an old navicular fracture for nearly 40 years, indicating that the friction between the tendon and the fracture end of the navicular bone accumulated over many years before the tendon ruptured, which also predetermined the very low incidence of this type of tendon rupture. In terms of treatment, the use of tendon displacement to alter the tendon pathway reduces the risk of re-rupture by reducing the friction between the tendon and the fracture end of the navicular bone. Internal fixation of the navicular fracture was not performed due to the patient’s work type and low requirements. Although the case was treated well, there are still many questions to be solved, whether intraoperative sex fracture fixation is needed in this case, whether perilunar dislocation requires repositioning, and whether perilunar dislocation via the navicular bone accelerates tendon rupture, as reported by Li Dawei et al [2] that simple old navicular fractures can lead to spontaneous tendon rupture, is still inconclusive.