Abstract: Compared with painted carbon steel, the degradation of the paint finishing system of magnesium alloys in salt spray was studied by Electrochemical Impedance Spectroscopy (EIS). The result showed that whether based on chromate conversion coating or not, the electrode process of the paint system on magnesium alloys was more inclined to be controlled by diffusion than that on carbon steel, and the slop of the diffusion line was less than 1. The possible reason was given. It was proved that inductance unit existed in the finishing system with chromate conversion coating. The coating resistance and capacitance were calculated with all EIS data. The finishing system with chromate conversion coating absorbed much more water and corroded more severely than that without chromate conversion coating. The calculated values were not fixed but varied, and the more severely the coating degraded, the more widely the value ranged.