The X56 steel samples was corroded in the medium of salt water solution at the conditions of CO2 partial pressure POM 0.5 to 2.0 MPa, temperature 80 °C and flow rate 1.4m/s. Corrosion weigh loss, composition and structure, morphology and phase of corrosion films of the samples were investigate by SEM, EDS, XRD and XPS. The results indicated that the corrosion degree was accelerated with increasing POM ? The intense localised corrosion occurred on the surface of samples. The corrosion films mainly comprised of FeCO3 and complex phase products (Fe, Ca....)COj. There exists serious pitting on the metal substrates under the corrosion film. The theoretic and experimental analyses indicate this is caused by existed micropores or micro holes in films, which have the function of mass transportation. The X56 steel samples were corroded in the medium of salt water solution at the conditions of CO2 partial pressure POM 0.5 to 2.0 MPa, temperature 80 ° C and flow rate 1.4 m / s. Corrosion weigh loss, composition and structure, morphology and phase of corrosion films of the samples were investigated by SEM, EDS, XRD and XPS. The results indicated that the corrosion degree was accelerated with increasing POM? The intense localised corrosion occurred on the surface of samples. The corrosion films mainly comprised of FeCO3 and complex phases There exists serious pitting on the metal substrates under the corrosion film. The theoretic and experimental analyzes indicate this this by caused by microp micropores or micro holes in films, which have the function of mass transportation .