Abstract: The bolt used for connecting internal piston and slide valve fractured in service only for 2 500 h.The fracture reasons were analyzed by means of chemical compositions analysis,macroscopic observation,metallographic examination and scanning electronic microscope fracture analysis.The results show that the main cause for the connecting bolt fracture was under the action of alternating stress,cracks formed prematurely from the stress concentration position of the tooth root caused by additional bending stress and propagated in fatigue form,which eventually caused the fracture of the connecting bolt.