The corrosion behavior of P110S low alloy steel in H2S/CO2 environment and the effect mechanism of corrosion products on it were studied. In this paper, the effect of P110S low-alloy steel on corrosion weightlessness experiment at different temperatures, microscopic SEM morphology observation, XRD, and ion selectivity experiments to explore the crystal structure of corrosion products and the effect of ion selectivity on corrosion behavior. The environment of the corrosion experiment is to simulate the corrosion conditions of different well depths in the field of an oil field in Northwest China. The partial pressure ratio of CO2 and H2S remains basically unchanged at 2.5. When the temperature is lower than 100 ℃, the corrosion product is mainly mackinawite type FeS, which is cation selective, can prevent anions from contacting the substrate, and plays a role in inhibiting corrosion. Therefore, the corrosion rate is low, about 0.15 mm/a. And it basically keeps unchanged with the increase of temperature, the corrosion product film on the sample surface is flat and does not fall. After the temperature reached 120 ℃, the corrosion rate increased sharply, and some corrosion products were transformed from mackinawite to pyrrhotite. The corrosion product film was cracked and peeled off due to the extrusion of the underlying corrosion product, and the sample was locally corroded. When the temperature is higher than 160 ℃, the corrosion products are all pyrrhotite type FeS, which is anion selective and cannot prevent the anions from contacting the substrate through the corrosion product film. Therefore, as the temperature increases, the corrosion rate gradually increases and becomes flat, reaching 3.6 mm/a. The corrosion behavior of low alloy steel in H2S/CO2 environment is closely related to the crystal structure and ion selectivity of the corrosion product. If the corrosion product is mackinawite type FeS, it has cation selectivity and can inhibit the corrosion and dissolution of the metal matrix. However, if the corrosion product is pyrrhotite type FeS, because of its anion selectivity, the corrosion and dissolution of the metal matrix cannot be suppressed.