Abstract: The corrosion behavior of oil casing steels 110H, L80-3Cr, L80-9Cr and L80-13Cr was studied in CO₂ and H₂S environment of heavy oil thermal recovery simulated by high-temperature and high-pressure autoclave. The results show that the uniform corrosion rates of four kinds of oil casing steels increased with the increase of temperature in solo CO₂ corrosion environment, and at 80 ℃ they decreased with the increase of Cr content in the oil casing steels. In the corrosion environment of CO₂ and H₂S coexisting the uniform corrosion rates of oil casing steels presented different rules with the change of CO₂ and H₂S partial pressure ratio p(CO₂)/p(H₂S). As the p(CO₂)/p(H₂S) increased at 80 ℃, the uniform corrosion rates of 110H and L80-3Cr increased gradually, but those of L80-9Cr and L80-13Cr firstly increased and then decreased. As the p(CO₂)/p(H₂S) increased at 150 ℃, the uniform corrosion rates of the all oil casing steels showed a tendency of first increase and then decrease. In the CO₂ and H₂S coexisting environment at 300 ℃ and 350 ℃, the corrosion resistance of L80-9Cr and L80-13Cr were much better than those of L80-3Cr and 110H.