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Al-Mg-Zn alloy has moderate strength, excellent corrosion resistance and an age-hardening response through precipitating T-Mg32(Al, Zn)49 phases and it is expected to be used as structural material in aerospace and various types of hulls. Sc is one of the most effective microalloying elements in aluminum alloys. The primary Al3Sc phase formed during solidification acts as a nucleating particle to effectively refine the as-cast microstructure of the alloy, and the secondary Al3Sc phase formed during homogenization significantly inhibits recrystallization. Al3Sc particles have excellent thermal stability and can effectively improve the weldability of alloys. Due to the high price of Sc, the method of Sc and Zr compound addition is widely used in industry. The Al3(Sc, Zr) phase formed by adding Sc and Zr at the same time has similar physical and chemical properties to Al3Sc and can greatly reduce the cost while retaining the strengthening effect. In this paper, four Al-Mg-Zn alloys with different Sc/Zr ratios were studied, and the influence of Sc/Zr ratio on the mechanical properties of Al-Mg-Zn alloys in aged state was studied. Transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to investigate the effect of Sc and Zr microalloying on the precipitation of T-Mg32(Al, Zn)49 phase, and the contribution of different strengthening mechanisms to yield strength was quantitatively calculated. The results showed that a large amount of T phase is dispersed in Al-Mg-Zn alloy with small size, and different Sc/Zr ratio has no significant effect on T phase. The main strengthening mechanism of Al-Mg-Zn alloy was the precipitation strengthening of T phase, which contributes 283~297MPa to the strength. The contribution of solid solution strengthening to the strength of the alloy was 33~40MPa. The mechanical properties of the alloy were improved by the addition of Sc and Zr, and the properties of the alloy were increased by about 30MPa through grain boundary strengthening. The tensile strength of 0.13Sc-0.14Zr alloys and 0.17Sc-0.1Zr alloys was 551~552MPa and the yield strength was 461~463MPa.