Using electrochemical impedance spectroscopy (EIS), the analysis of impedance models and water uptake of silicone-epoxy coating for samples subjected to various levels of cathodic protection (CP) were studied during coating aging process in 5 wt.％ NaCl aqueous solution. The results show CP is supposed to protect mild steel from corrosion for samples during coating degradation process since there is a cathodic current and the mild steel surface is polarized cathodically, no corrosion is expected to take place and no corrosion product is expected to accumulate at the interface. Two distinguishing impedance models are found to fit the spectroscopy of the sample with CP. The coating systems under CP have a smaller water diffusion coefficient, and the water diffusion coefficient increases with the increasing of the cathodic potential. A further water uptake process after the saturation period is presented.