In a shale gas well in southwest China, the formation mainly contains pyrite (FeS₂) and the formation temperature is 130℃ - 150℃ . Using oil based drilling fluid (containing 25% CaCl₂) , several failure accidents of guiding tools occurred in the drilling process, and a large number of cracks and serious corrosion cracking occurred on the outer surface of the antenna, which led to serious economic losses. In order to find out the failure reason, macro analysis, chemical composition analysis, metallographic structure analysis and corrosion test under simulated working conditions were carried out on the failed antenna. SEM and EDS were used to analyze the surface crack morphology and corrosion product morphology. The results show that the S element content of the failure antenna is higher. Antenna failure is mainly caused by chloride stress corrosion cracking, Cl^- destroys the passivated film on the surface of the antenna substrate forming pitting holes, and promotes the development of a little bit of corrosion under stress, leading to crack initiation. Cl^- penetrates into the corrosion product film, forming the acid environment, and causing the corrosion of Fe dissolution in the matrix to gradually expand crack, and finally resulting in antenna failure. It is suggested to reduce the inclusion content and choose the material with strong pitting corrosion resistance, improve the antenna structure design, reduce stress concentration, avoid large residual stresses as much as possible, and deoxidize the drilling fluid. Keep the pH of the drilling fluid above 10 and reduce the Cl^- content when drilling through the pay zone to prevent synergistic action of oxygen and chlorine from accelerating stress corrosion.