Abstract: The corrosion behavior of H2S resistant casing steel BG110S in NACE solution saturated with H2S was investigated by means of electrochemical methods, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The results show that the test steel reacted with hydrogen sulfide in NACE solution saturated with H2S at 25 ℃, and double layer corrosion films, whose outer layer was mackinawite (FeS) and inner layer was Cr and Mo sulfide, formed on the surface of test steel. The corrosion films could resist further corrosion reaction between the test steel and H2S, and decreased hydrogen permeation flux, which improved the H2S resistant property of the test steel.