Abstract: Based on the anslysis of fracture morphology, chemical composition, mechanical properties, microstructure and corrosion products, the cracking causes for casing coupling in an oil well were analyzed. The results show that the main cause for coupling cracking was the stress corrosion cracking because of hydrogen sulfide. The sulfide corrosive media, which the couplings were exposed to in the well, led to serious pitting corrosion on the surface of couplings, and then etch pits formed, finally the coupling cracked under the action of both tensile stress and media environment. In order to prevent the recurrence of the accident, it is suggested that sulfide-resistant pipe and premium connections should be used in this area.