The deterioration process of the epoxy phenolic coated tinplate in functional beverage was studied by electrochemical impedance spectroscope (EIS). The surface morphology was characterized SPM and SEM. The results showed that corrosion pits were observed on the surface after exposing the coating system to functional beverage for 364 days. The breakages of the organic coating lead to the corrosion of substrate metal. A new parameter p extracting directly from the Bode plot was presented to characterize the deterioration degree of the coating system. Using p value as the discrimination indicator of the deterioration degree of organic coating was presented in this paper.