As mild steels are the most widely used structure materials, it is no doubt that the study on their atmospheric corrosion mechanism is of great significance. Traditional study on atmospheric corrosion of mild steels emphasized more on on-site experiments and data collection and physical analysis. Relatively, application of electrochemical researching methods has not been frequently used in this field. Electrochemical impedance spectroscopy (EIS) method has been highly valued by electrochemists due to its multiple advantages. However, it has been less used in study of corrosion process of mild steels after the rust has been generated. As a matter of fact, during the service life of mild steel in atmospheric environments, the rust period is much longer than naked steel period due to the low corrosion resistance of these steels. Therefore, it is more meaningful to study the corrosion behavior of steels with rusts both for development of new weathering steels and for the prediction of the service life of weathering steels. In this work, the corrosion behavior of mild steel Q235 under simulated industrial atmospheric corrosive condition was monitored and studied by EIS method, with a focus on the evolution of cathode reactions. The result showed that there existed a critical condition of electrolyte film for corrosion rate of mild steel Q235 both for the substrate and rusted surface during wet-dry cycles. Comparison and analysis showed that, two parallel reactions, the reduction of oxygen and the reduction of rust, existed at the cathode during the corrosion process. With the increase of the wet-dry cycles, the activity of oxygen reduction tended to decrease, and almost disappeared in the end; while the activity of rust deduction tended to increase and became the main cathode reaction in a short period. In each wet-dry cycle, the corrosion rate first increased then decreased with the decrease of the electrolyte layer thickness, which was caused by the two synchronous and opposite effects on the corrosion reactions.