This paper presented the basic principle of EIS technique, and gave an example of its application to the evaluation of the anti-corrosion performance of the composite protective coating and polyurethane emulsion for iron relic. Electrochemical impedance spectroscopy (EIS) technique is one of the most important electrochemical methods used extensively for evaluating the anti-corrosion performance of coat. The results showed that the composite protective coating performed better than polyurethane emulsion, which demonstrated that EIS offers a rapid and efficient method for the study of protective coating on metallic relic, and it is promising in the field of relic protection.