Abstract: The spring pull rod adjustable fork joint of an aircraft fly by wire control system fractured during operation. The causes of fracture on pull rod joint were analyzed by means of macro and micro analysis and metallographic examination. The results show that the fracture of pull rod joint was fatigue fracture, and the root cause of fracture was that the microstructure was not uniform and widmanstatten structure appeared at the same time, which seriously reduced the fatigue resistance of the joint, led to the initiation and propagation of fatigue cracks. Uneven stress on the pull rod accelerated the propagation of fatigue cracks.