Heat treatable Al–Zn–Mg alloys can be strengthened by the precipitation of second phase particles. In this article, four Al–Zn–Mg-based alloys with different Cu contents have been prepared. The effects of Cu content on the aging behaviors of the whole precipitation process of the prepared Al–Zn–Mg alloys have been investigated using hardness tests, transmission electron microscope, selected election diffraction patterns, and their simulations. It has been shown that a certain amount of Cu promotes a rapid hardening response at the early stage of aging and the nucleation of both GP-II zones and the η′ precipitates. However, the addition of Cu above 3 wt% in alloys would decrease the accelerating effect of the transition from the GP-II zone to the η′ precipitates. The η precursor has been observed in Cu-containing alloys and it can retard the transition to the stable phase.