Objective To investigate the effects of metal coatings (Al-Zn alloy coating and pure zinc coating) on the corrosion resistance of prepainted sheets. Methods Metallographic observation, scanning electron microscopy (SEM), energy dispersive spectrum(EDS), cyclic corrosion test and electrochemical impedance spectroscopy(EIS) were used to analyze and compare the corrosion resistance of the two kinds of prepainted sheets which had the same thickness of metal coating. Results In corrosion test of 250 cycles (a total of 2000 h), serious red rust appeared on the prepainted sheets of pure zinc coating, and no obvious corrosion on the surface of Al-Zn alloy coating prepainted sheets. In the EIS test, the initial impedance value of the two kinds of prepainted sheets were both about 2×109 Ω·cm2. The impedance value of prepainted sheet of pure zinc coating sharply declined to 1.165×108 Ω·cm2 after 2 days immersion, while undergoing slowly descending, the impedance value of Al-Zn alloy coating prepainted sheet sharply declined from 6.979×108 Ω·cm2 to 2.984×107 Ω·cm2 after 15 days immersion. The equivalent circuit fitting parameters for EIS showed that charge transfer resistance of Al-Zn alloy coating prepainted sheet was one order of magnitude higher than that of Zn coating prepainted sheet, and the corresponding double-layer capacitance was one order of magnitude lower. SEM and EDS tests showed that Al-Zn alloy coating was composed of inner alloy layer and outer two-phase structure which consisted of alumina-rich phase and zinc-rich phase, while Zn coating was composed of inner alloy layer and outer pure zinc layer. Conclusion Prepainted sheet of Al-Zn alloy coating has better corrosion resistance than that of pure zinc coating due to the unique two-phase structure of Al-rich and Zn-rich.