Abstract: Cracking occurred on the outer surface of the super 13Cr martensitic stainless steel oil tube coupling in a certain oil field in the west. The causes of the cracking were analyzed using methods such as macro observation, chemical composition analysis, metallographic examination, and mechanical property tests. The results indicate that the type of cracking was sulfide stress corrosion cracking, and the cracks originated from the outer surface of the coupling. The presence of sulfides in the high-temperature and high-pressure oil casing annular protective fluid was the main environmental factor leading to the cracking of the coupling.