Abstract: Nitrogen injection is a chief technique to efficiently improve oil recovery rate in carbonate reservoirs now. With the promotion of nitrogen injection technology, more and more corrosion failures were found in wellbores. Based on the studies of nitrogen injection technique, media and production, we analyzed the corrosion mechanism and influencing factors of down-hole strings. Results showed that the mixture of high-concentration oxygen and high salinity, week-acidic water created electrochemical corrosion and erosion corrosion, which caused whole corrosion inside pipe and pitting corrosion outside pipe wall. Corresponding countermeasures were proposed in this paper combining with field applications.