Water soluble chitosan was degraded to different molecular mass products by hydrogen peroxide at first, and then molecular mass of the degradation products were measured using terminal group analysis method. The inhibitive properties of water soluble chitosan and its degradation products for mild steel were studied by mass loss method and potentiodynamic polarization measurement. The inhibitive mechanisms were analyzed by polarization curves and ellipsometry method. The results show that with the increasing of hydrogen peroxide quantity the degradation products molecular mass were decreased. It was found that the highest inhibition effect of water soluble chitosan and its degradation products was obtained at 400 mg/L and the lower the molecular weight, the higher the inhibition efficiency, which was up to 64.35%. The results of polarization curve method show that the water soluble chitosan and its degradation products have similar inhibition tendency measured by mass loss method and are cathodic corrosion inhibitors. Ellipsometry experimental results indicated chitosan took place multi-layer absorption on carbon steel surface and inhibition film became thicker with molecular mass decreasing. The water soluble chitosan with low molecular mass are green corrosion inhibitor for mild steel in seawater.