Abstract: The M50 bolt in service in a certain marine environment broke. The causes of bolt fracture were analyzed using macroscopic observation, chemical composition analysis, scanning electron microscopy and energy spectrum analysis, X-ray diffraction analysis, secondary mass spectrometry analysis, metallographic examination. The results show that long-term service of bolts in humid marine environments led to electrochemical corrosion, resulted in the formation of pitting corrosion pits on the bolt surface and the release of hydrogen ions. There was stress concentration at the root of the bolt, and hydrogen ions accumulated in the root thread area, ultimately led to stress corrosion cracking of the bolt.