Objective To investigate the influence process of rust layer on iron corrosion. Methods The corrosion behavior of carbon steel in diluted NaCl solution was studied by weight loss method, surface analysis and electrochemical techniques. Results Experimental results showed that the initial product γ-FeOOH worked as an oxidant and was easily reduced to Fe₃O₄. Hence, a double-layer rust, consisted of a thin yellow outer layer ( γ-FeOOH layer) and a thick black inner layer ( Fe₃O₄layer) , emerged rapidly on metal surface. The outer rust layer tended to facilitate the cathodic reaction via reduction of γ-FeOOH. The inner rust layer worked as a large area cathode and dissolved oxygen was directly reduced on its surface. Conclusion Therefore, the corrosion rate of carbon steel was accelerated quickly and was finally determined by the limiting diffusion rate of dissolved oxygen. Anti-corrosion measures for iron should be aimed to reduce the promotional effect of the rust layer on metal corrosion.