Through the analysis of the electrochemical impedance spectroscopy（EIS）, the resistance capacitance and the characteristic frequency of the epoxy glass flake coating, the protective mechanism were studied. Results show that in the early stage of immersion, the epoxy glass flake coating has good shielding effect, which can prevent the infiltration of water molecules and the corrosive medium; in the immersion medium, epoxy glass flake coating fails due to water saturation, resulting in corrosion spread into the steel substrate surface, electrochemical reaction and corrosion, but because the corrosion product fills pores and layered glass flake corrosion medium makes in depth growth path, which effectively lowers the corrosion rate of coating; in the late period of immersion, the epoxy glass flake coating is quickly ineffective, and the increase of the corrosion product causes the expansion of the coating. The corrosive medium directly has an electrochemical reaction with the metal substrate, the coating is completely ineffective, and the protection ability becomes very poor.