In this research, pre-aging treatment has been performed on our newly developed Al-5.2Mg-0.45Cu-2.0Zn alloy. Results show that pre-aging treatment not only improves the stability of the alloy sheet at room temperature, which avoids the paint-baking softening, but also enhances the age hardening response during subsequent artificial aging at 180℃. The microstructure of the T4 sample in peak aged condition consists of coarse lath-like T-Mg32(AlZn)49 phase and needle-like S-Al2MgCu phase. However, the microstructure of the T4P sample in peak aged condition only consists of high density of finer and equiaxed T phase with the S phase disappeared. Unstable clusters formed during natural aging dissolve when imposed on 180℃, which retards the precipitation process of the alloy. Stable clusters formed during pre-aging treatment serves as the nucleis of the T phase during aging at 180℃, which enhances the age hardening response of the alloy.