Abstract: The high temperature and high pressure dynamic reactor was used to simulate the actual working conditions of CO₂ and H₂S coexisted environment in oil and gas field. The corrosion behavior of 20 steel at different CO₂ partial pressures (0.01, 0.05, 0.21, 0.50 MPa) and H₂S partial pressures (0.000 3, 0.003, 0.01, 0.05 MPa) and the degree of influence of CO₂ and H₂S partial pressure on uniform corrosion and pitting were studied by corrosion tests. The results show that under H₂S partial pressure of 0.003 MPa, when CO₂ partial pressure was 0.01, 0.05 MPa, uniform corrosion of 20 steel mainly occurred; with increasing CO₂ partial pressure, pitting became serious. Under CO₂ partial pressure of 0.05 MPa, when H₂S partial pressure was 0.000 3 MPa, 20 steel's surface corrosion was slight; with increasing H₂S partial pressure, the corrosion was intensified, but still uniform corrosion mainly occurred. H₂S and CO₂ partial pressure had a very significant impact on 20 steel's uniform corrosion rate and pitting rate; H₂S partial pressure had a relatively great effect on uniform corrosion, and CO₂ partial pressure had a relatively great effect on pitting.