Objective To study the effect of partial pressure of CO2 on the corrosion in CO2/H2S environment, in order to guide the material selection of subsea pipeline. Methods Corrosion simulation test was carried out in the high-pressure high-temperature autoclave, the weight of the specimen was tested before and after corrosion simulation experiment, then the corrosion rate was calculated, and the morphologies and chemical composition of corrosion product scales were characterized by SEM and XRD. Results When the ratio of CO2/H2S was high (1200), at the partial pressure of CO2 of 0.3 MPa, 0.5 MPa, 1.0 MPa, the corrosion rates were 1.87 mm/a, 3.22 mm/a, 5.35 mm/a, respectively, and the corrosion rate increased almost linearly with the increase of CO2 partial pressure. When the ratio of CO2/H2S was low (200), at the partial pressure of CO2 of 0.3 MPa, 0.5 MPa, 1.0 MPa, the corrosion rates were 3.47 mm/a, 3.64 mm/a, 3.71 mm/a, and the influence of CO2 partial pressure on corrosion rate was not significant. When the ratio of CO2/H2S was high (1200), the main chemical composition of corrosion product scale was FeCO3, the corrosion was controlled by CO2; the corrosion product scale was compact under low partial pressure of CO2, the scale was easy to be ruptured, and the protection of the substrate was decreased, so the corrosion rate increased with the increase of partial pressure of CO2. When the ratio of CO2/H2S was low (200), the main chemical composition of corrosion product scale was FeS, the corrosion was controlled by H2S; the corrosion product scale was compact under different partial pressure of CO2, so the corrosion rate was relatively low, and there was no significant increase with the increase of partial pressure of CO2. Conclusion the effect of partial pressure of CO2 on the corrosion in CO2/H2S Environment was closely related to the ratio of CO2/H2S. We need to consider the effects of partial pressure of CO2 and the ratio of CO2/H2S on corrosion when we select materials for subsea pipeline.