In recent years, the corrosion problem of steels under supercritical CO₂/H₂O system in oil/gas production has got more and more attention. The temperature and pressure of some oil wells in China usually exceed 120 ℃ and 100 MPa, where CO₂ is in supercritical state. To transportation easier and cost reduction, the oil/gas in pipelines is usually pressured to a high pressure, normally causes CO₂ in supercritical state. The supercritical CO₂ corrosion environment includes CO₂-saturated water and H₂O-saturated CO₂ phases. Moreover, corrosive ions such as Cl^- usually exists in CO₂ corrosion environment, however the influence of Cl^- on corrosion of carbon steel in supercritical CO₂-saturated NaCl solution and NaCl solution-saturated supercritical CO₂ are investigated limited. The corrosion behaviors and corrosion rates of X65 carbon steel exposed in supercritical CO₂-saturated 3.5%NaCl solution, supercritical CO₂-saturated deionized water and NaCl solution-saturated supercritical CO₂ systems were investigated. SEM, EDS and XRD were used to analyze the morphology and characteristic of corrosion product scale on the steel surface. The results show that the addition of Cl^- in supercritical CO₂-satureated water significantly increased the corrosion rate of X65 steel, and modified the FeCO₃ grain morphology. The average corrosion rate of X65 steel in NaCl solution-saturated supercritical CO₂ was much lower than in supercritical CO₂-saturated NaCl solution, but in supercritical CO₂ phase X65 steel suffered serious localized corrosion. The corrosion process of X65 steel in supercritical CO₂-saturated NaCl solution could be divided into three stages: the first was the active dissolution stage, the surface of X65 steel was corroded inhomogeneous due to the competitive adsorption between Cl^- and H₂CO₃, HCO₃^-, and Fe₃C as well as some lumpish matrix were residued on steel surface; the second was the initiation stage of FeCO₃ precipitation, Cl^- postponed the precipitation of FeCO₃, the FeCO₃ scale formed in this period was incomplete, and increased the area of cathodic reaction subsequently the corrosion rate; the last was the protective stage of FeCO₃ corrosion scale, the corrosion product scale formed in this period was denser and provided better protectiveness to X65 steel matrix, however Cl^- could pass this scale and reach the scale/matrix interface, resulted in the corrosion rate of X65 steel keeping at a higher value than in deionized water environment. The corrosion model of normal pipelines was developed to better understand the corrosion mechanism in supercritical CO₂-saturated Cl^--containing solution.