Abstract: Chain pin shaft in one aero navigating system were fractured in service. A cracked pin shaft and a fractured one were analyzed,through visual inspection,fracture analysis and presentation quality check. The results showed that there existed original defects in the pin shaft surface,and the pin shaft bore bend and shear load which wear of chain plates during long-term service enlaryed the dearomce of the plates,original defects and wear of chain plates caused the chain pin shaft fatigure failure.