Abstract: To study the effect of photo degradation on the adhesion at coating/steel interface, the acrylic polyurethane varnish (APV) coatings on the steel were aged under outdoor field environments in the cities of Wuhan and Lhasa and the indoor artificial weathering environments. Pull-off adhesion measurement and electrochemical impedance spectroscopy were applied to investigate the adhesion of dry coating and the wet adhesion at coating/steel interface, respectively. It is found from pull-off measurements that the adhesion decreases with aging time increasing. The same trend is observed in the change of coating delamination area, which is deduced from EIS parameters. That is, the delamination area enlarges with aging time. Based on EIS results, it is found that the delamination area produced by fluorescent UV/condensation weathering device is larger than that by xenon lamp exposure and weathering equipment. This result is also verified by comparing the samples respectively exposed in Lhasa with higher irradiation intensity and Wuhan with less irradiation intensity, which indicates that the enhancement of UV intensity impairs the wet adhesion of coating. Thus, photo degradation undermines the adhesion at the coating/steel interface and further accelerates the underneath corrosion of coatings.