The mechanical properties and microstructures of the Al-6.02 wt% Zn-1.94 wt% Mg alloy were researched by tensile test, hardness test, optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The results showed that the peak aging time shortens and the tensile strength slightly enlarges with the increase of pre-stretching. Synchronously, the cleavage fracture of the pre-stretched samples from 0% to 6% was increasingly apparent, and their elongation got lower. The precipitation sequence of 4% pre-stretched alloy during aging treatment at 120 °C was supersaturated solid solution (SSS) → GPI zone → η′ phase → η (MgZn2) phase. Rationally, the under-aged GPI zone and the over-aged η phase had smaller contribution to the intensity as against the η′ phase of the peak-aged state. The GPI zone was beneficial to elongation, which improved the fracture morphology.