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The precipitation of MgZn2 phase in aging process of Al-Zn-Mg-Cu alloy forging was characterized qualitatively after transmission electron microscope (TEM) observation, X-ray diffraction phase analysis and the exact measure of lattice parameter. And, the precipitation of the second-phase in aging process was simulated after test the resistivity of the alloy in a continuous heating process and delayed time processing. The results show that when heating in the same rate, the alloy resistivity increases with temperature first, a mutation point appears at 110℃, and the resistivity reaches its peak at 120℃. Then, the resistivity decreases and achieves a minimum at about 170℃. Afterwards, it increases again. The change of resistivity results from a combined effect of the temperature and phase transformation. The volume precipitation of η′ phase dominates at about 120℃, and the resistivity of forgings decreases accordingly. When the specimen cools in a furnace at 240℃, the phase transformation finishes basically and the alloy content of Al-matix keeps stable, thus, the alloy resistivity depends on the temperature only. It decreases with temperature and time since the end of precipitation. The higher the temperature is, the bigger it will be. With a big descending rate of the resistivity curve, the time to reach the stable value becomes shorter.