The effect of Si additions on microstructure, mechanical properties and localized corrosion behaviors of Al-Zn-Mg-Cu alloy was investigated by means of mechanical tensile and exfoliation corrosion testing combined with optical microscopy(OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that the strength of the alloys first increases and then decreases with the increase of Si contents, when Si content is 0.040% (mass fraction), the strength of the alloy reaches the peak value; The anisotropic mechanical properties of the alloys first decreases and then increases with the increase of Si contents, when Si content is 0.025%, the alloy has the minimum anisotropy. Concomitant with this increase in Si contents, a transition in fracture mode was observed from predominantly transgranular dimpled rupture to predominantly intergranular dimpled rupture of transverse. Compared with the change of the longitudinal strength of the alloys, the Si contents have a significant effect on the transverse strength of the alloys.