Corrosion of oil and gas pipeline steel in the marine environment is one of the important factors that damage the integrity of oil and gas pipelines. The effects of different HCO〖_(3)^-〗 concentrations in simulated ocean solution on the electrochemical corrosion behavior of X100 steel were investigated by using potentiodynamic polarization and AC impedance technique. The results showed that the corrosion current density, the corrosion pits number and area of X100 steel increased with increasing of HCO〖_(3)^-〗 ion concentration. In addition, when HCO〖_(3)^-〗concentration was 0.1 and 0.5 mol/L, transition passivation zone appeared in the anodic polarization curve of X100 steel, and unstable product film was formed on the metal surface.