The work aims to study the galvanic corrosion behaviors of carbon steel/copper alloy couple under epoxy coating and the degradation process of the whole and local area of the coating exposed to seawater. The current density distributions of the WBE and the EIS evolution of the coating on WBE surface were studied by WBE and EIS techniques. Meanwhile, the EIS characteristics of the coating on carbon steel and copper alloy surface were also compared and analyzed. It showed that the anodic current peak first appeared at the local area of carbon steel, while the cathodic current peaks with larger current densities appeared at the edge of the copper alloy area. After 122 h of immersion, the impendence value of the coating on copper alloy surface was obviously lower than that of the coating on carbon steel surface, and the Warburg impedance behavior could be observed in the EIS of the coating on copper alloy surface. The current of the single steel electrode changed from cathodic to anodic when the WBE was immersed for 456 h. In conclusion, when the carbon steel and copper alloy under epoxy coating were electrically coupled in seawater, the copper alloy was protected by carbon steel from corrosion but the degradation processes of the coating on copper alloy surface was accelerated by cathodic delamination. During the degradation processes of the epoxy coating, the local defect area of the coating on carbon steel surface became the anodic area and the main cathodic area was concentrated at the edge of copper alloy area, which was related to the “competition effect” for dissolved oxygen. The reason of the polarity reversals from cathodic to anodic of the single carbon steel electrode may be that the metal was corroded because of cathodic delamination.