Abstract: The connecting screw between the adapter ring and the engine mounting edge of a certain type aircraft was broken. The causes of screw fracture were analyzed by macroscopic observation, chemical composition analysis, scanning electron microscopy and energy spectrum analysis, metallographic examination, cross groove depth detection and mechanical property test. The results show that the corrosive medium containing chlorine element was deposited at the cross slot of the screw during the service of the aircraft, and the depth of the cross slot of the screw exceeded the standard requirement, which caused stress concentration in this part. Under the combined action of the two, the stress corrosion cracking of the screw occurred at the root of the cross slot, and the crack extended from the root of the cross slot to the outer surface, which eventually led to the fracture of the screw.