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Two kinds of alloys Al-Zn-Mg-Cu and Al-Zn-Mg-Cu-0.15% Er were prepared by steel mould casting. The existence and form of the Er in the as-cast, homogenized, deformed and aged microstructures of the Al-Zn-Mg-Cu alloy were investigated by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), differential scanning calorimetry (DSC), transmission electron microscopy(TEM), X-ray diffractometry (XRD). In addition, its effect on the tensile properties of the alloy at room temperature has also been discussed in detail. The results show that trace element Er can refine the as-cast microstructure of Al-Zn-Mg-Cu alloy to a certain extent, but this refinement is limited. Er exists in the alloy mainly in the form of a ternary Al8Cu4Er phase, which forms during the solidification and segregates at the grain boundaries, and often occurs between eutectic networks. Al8Cu4Er has a high melting point and poor plasticity and is therefore easily crushed during the thermal deformation of the alloy. Al8Cu4Er fragments may cause cracks and may also induce recrystallization nucleation, which will eventually have an adverse effect on the comprehensive properties of the alloy.