Abstract: The effect of the content of molybdenum as an alloying element on the corrosion resistance performance of weathering steel was investigated by microstructure observation, cycle immersion corrosion test, rust layer micro-analysis and electrochemical impedance spectra techniques. It is found that molybdenum can promote the formation of bainite in the steel and the rust layer denser. The rust layer has a better protection on the steel substrate and increases the corrosion resistance of the steel. With the increasing of molybdenum content in the steel, the corrosion potential of the rust layer on the steel shifts positively, the self-corrosion current density decreases, the anodic dissolution of the rust layer is impeded, which makes the rust layer better protective.