With epoxy coal tar as the coating material, the electrochemical corrosion behavior of Q235 with different kinds of bulk coating holidays has been investigated with EIS （Electrochemical Impedance Spectroscopy） in a 3.5vo1% NaCI aqueous solution. The area ratio of bulk coating holiday to total coating area of steel is 4.91%. The experimental results showed that at free corrosion potential, the corrosion of carbon steel with disbonded coating holiday is heavier than that with broken holiday and disbonded ＆ broken holiday with time; Moreover, the effectiveness of Cathodic Protection （CP） of carbon steel with broken holiday is better than that with disbonded holiday and disbonded ＆ broken holiday on CP potential -850 mV （vs CSE）. Further analysis indicated that the two main reasons for corrosion are electrolyte solution slowly penetrating the coating, and crevice corrosion at steel/coating interface near holidays. The ratio of impedance amplitude （Z） of different frequency to minimum frequency is defined as K value. The change rate of K with frequency is related to the type of coating holiday.
