The on-site test of CO2 flooding in Y oilfield has achieved good results, but the corrosion problem of CO2 flooding casing is severe. Up to now, a total of 28 damaged casing wells have been discovered, resulting in imbalanced injection production well networks, low water flooding utilization, inability to extract remaining reserves, and difficulties in obtaining dynamic monitoring data, leading to increased measures and management costs, seriously affecting the stable production and development benefits of the oilfield. In response to the serious corrosion problem of CO2 flooding casing, non-destructive testing, mechanical performance testing, metallographic examination, scanning electron microscopy analysis and other methods were used, combined with macroscopic and microscopic morphology analysis of casing fractures and on-site gas sample testing. The cause of casing damage was identified as hydrogen sulfide stress corrosion cracking, and corresponding protective measures were proposed to prevent hydrogen sulfide corrosion of gas injection well casing, which has certain guiding significance for CO2 flooding development.