Abstract: The corrosion behavior and mechanism of J55 casing steel were studied through uniform and local corrosion tests in simulated downhole environment and electrochemical test analysis.The results show that the CO₂ uniform corrosion rate of J55 casing steel increases gradually with the increase of temperature.When the temperature reaches 60 ℃, the corrosion rate reaches the maximum value of 1.0175 mm/a, and the local corrosion is the most serious.SRB and TGB significantly increase the uniform corrosion rate and local corrosion degree of J55 casing steel.In CO₂ corrosion environment, both the cathode and anode of J55 casing steel show activation reaction, and the corrosion current density under self-corrosion potential is controlled by the cathodic reaction process.When the temperature reaches 60 ℃, the dynamic resistance of J55 casing steel reaches the minimum value and the self-corrosion current density reaches the maximum value.SRB and TGB significantly reduce the self-corrosion potential of J55 steel in CO₂ corrosion environment, while can promote the cathodic hydrogen depolarization process of CO₂ corrosion, so the anodic dissolution of J55 steel is accelerated.