The reduction behavior of rust layer formed on carbon steel and on Cr-containing steels has been investigated in flowing thin electrolyte layer with 3.5% NaCl solution, via potentiodynamic polarization curve, EPMA, Raman spectroscopy, XRD, and XPS. The cathodic potentiodynamic polarization curves of carbon steel and Cr-containing steels were carried out in flowing thin electrolyte layer with vacuum environment, for eliminating the reduction reaction of oxygen. Results show that the cathodic reaction of Cr-containing steels was inhibited by concentration of Cr in the inner rust layer, and the reduction of rust layer formed on Cr-containing steels was also inhibited. The constituent of rust layer formed on carbon steel was the same with that of Cr-containing steels. However, the amount of Fe3O4 in the rust layer formed on carbon steel was higher than those formed on Cr-containing steels after cathodic potentiodynamic parization curves. It indicates that the reduction of Fe3+ was suppressed for Cr-containing steels. Cr exited in the form of Cr3+ in the corrosion product, participating in the transformation of corrosion product.