A series of study was conducted to investigate the corrosion behavior of 2Cr13 stainless steel and X65 carbon steel exposed to water-saturated and water-unsaturated dense CO2 environments with SO2 and O2 impurities in the conditions of 25 °C and 50 °C, 8 MPa. Each test lasted for 48 h. The cross impacts of CO2 phase (liquid phase and supercritical phase), water contents and steel types on the corrosion behavior were elaborately discussed. In water-saturated impure dense CO2 with SO2, SO2/O2 systems, the impacts of single impurity of SO2 and multiple impurities of SO2 and O2 on accelerating general corrosion for both 2Cr13 and X65 steels was much stronger in the supercritical CO2 system than in the liquid CO2 system. At water-unsaturated conditions, 2Cr13 stainless steel exhibited better resistance to the general corrosion attack in liquid and supercritical CO2 with SO2 in comparison with X65 carbon steel, but suffered high risk of localized corrosion in liquid CO2 mixtures. The 2Cr13 and X65 steels could bear higher amounts of water in supercritical CO2 with SO2 systems (∼1300 ppm H2O) than in the liquid CO2 with SO2 systems (<300 ppm H2O).