Abstract: The effect of HCO₃^- concentration on CO₂ corrosion was investigated by polarization measurement of potentiodynamic scans and weight-loss method. Under the conditions of high pressure and high temperature, the corrosion rate of steel X65 decreased with the increase of HCO₃^- concentration, while pH of solution increased. SEM, EDS, and XRD results of the corrosion scales indicated that the typical FeCO₃ crystallite was found at low HCO₃^- concentration but Ca(Fe,Mg)(CO₃)₂ was found at high HCO₃^- concentration. Ca²⁺ and Mg²⁺ are precipitated preferential to Fe²⁺ at high pH value. Potentiodynamic polarization curves showed that the cathodic current density decreases with the increase of HCO₃^- concentration at low HCO₃^- concentration. When the HCO₃^- concentration reaches 0.126 mol/L, increasing HCO₃^- concentration promotes cathodic reactions. Anodic behavior is an active process at low HCO₃^- concentration and the anodic current density decreases with the increase of HCO₃^- concentration. An evident active-passive behavior is exhibited in anodic process at 0.126 mol/L HCO₃^-