Abstract: Analyzed in the paper is the pitting corrosion on N80 3Cr anti-corrosion casing by means of macroscopic analysis，metallographic analysis，energy specetum analysis and SEM analysis.The analysis results show that the pitting corrosion occurred on the thread belongs to oxygen corrosion.Corrosion of a coupling is mainly caused by dissolved oxygen from the residual water on the gearing face in between the coupling and the protector，and the speed of pitting corrosion is accelerated with the presence of Cl^-.It is suggested to improve the field storage environment or select materials with good corrosion resistance according to the working condition.