Abstract: Corrosion tests were conducted on 20CrMnMo steel and 20CrNiMo steel for packer slip in the high-temperature, high-pressure and CO₂/H₂S environment simulating deep/ultra deep well conditions. The corrosion behavior of the two materials were compared and studied by means of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electrochemical testing. The results show that under the simulated deep/ultra deep well conditions, the corrosion form of 20CrMnMo steel was typical general corrosion, while the corrosion form of 20CrNiMo steel was pitting corrosion. The corrosion products on the surfaces of the two materials had a double-layer structure, and the outer corrosion products were composed of loose flocculent FeS and granular FeCO₃. The inner corrosion product of 20CrNiMo steel was composed of FeCO₃, while the inner corrosion product of 20CrMnMo steel had a small amount of Cr(OH)₃ in addition to FeCO₃. The existence of Cr(OH)₃ could effectively improve the protection of the inner corrosion product film of 20CrMnMo steel, so that the corrosion resistance of the steel was obviously better than that of 20CrNiMo steel.