A comparative study of the corrosion properties of an AlZnMgCu alloy subjected to simulate age-forming (SAF) and traditional aging was performed. The samples pretreated at 160 °C for 24 h using SAF and traditional aging, were immersed in 3.5 wt% sodium chloride solution for up to 96 h, the process of which was monitored by in situ electrochemical impedance spectroscopy (EIS). Optical profilometry measurement shows that the sample subjected to SAF has deeper pits and larger localized corrosion compared with the traditionally aged sample. EIS results suggest that corrosion rate changes with increasing time of immersion, and age forming makes the AlZnMgCu alloy hard to form effective passive film. The larger precipitate and wider precipitate free zone of the sample subjected to simulate age forming were thought to be responsible for the poor corrosion resistance during the immersion test. Additionally, these results suggest that stress during age-forming increases the grain aspect ratio, which is harmful to intergranular corrosion resistance of the AlZnMgCu alloy.