The variation of microstructure and mechanical properties along the cross section of squeeze cast Al-5Mg-3Zn-1Cu-1Sialloy is studied under both squeeze-cast and peak-aged condition (470 °C × 24 h + 125 °C × 24 h). For squeeze-cast alloy,along the depth direction, second dendrite arm space and the percentage of -Al increase significantly, while the volumefraction of T-Mg32(AlZnCu)49 and Mg2Siphase showed a dramatic decrease. The ultimate tensile strength of squeeze-castalloy slightly decreased from 357.04 MPa for the surface layer to 344.21 MPa for the central layer, yield strength droppedfrom 296.86 MPa to 241.03 MPa, but the elongation almost tripled from 1.42% to 3.78%. After peak aging, the ultimatetensile strength of surface layer and central layer reached 538.45 MPa and 504.03 MPa, yield strength reached 447.46 MPaand 442.65 MPa, and the elongation reached 5.23% and 3.97% respectively. Meanwhile the difference of ultimate tensilestrength and elongation was increased, but that of yield strength reduced after aging treatment. Precipitation strengtheningis the main strengthening mechanism while the improvement in ductility is due to the dissolution of T-Mg32(AlZnCu)49 andthe morphological change of eutectic Mg2Si.