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The enhanced phase Mg₂Si in the magnesium alloy can significantly improve the hardness, wear resistance, especially high temperature creep resistance. But the Mg₂Si phase of the as-cast hyper-eutectic Mg-Si alloy will seriously cut the alloy matrix because of the coarse angular shape of the primary Mg₂Si and complex Chinese script morphology of the eutectic Mg₂Si. In order to improve the properties of Mg-2.5Si-4Zn alloy, the modification experiments of adding Er/Er-Ba were performed and the effects on the alloy microstructure and Mg₂Si phase were investigated by the optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray diffractometer (XRD). The mechanical properties were measured and analyzed by the computer-aided electric-loaded tensile tester. The results show that with the addition of 0.6wt% Er to the Mg-2.5Si-4Zn alloy, the primary Mg₂Si in the microstructure transforms from a coarse dendritic shape into a regular tetragonal block shape, while the eutectic Mg₂Si transforms from a coarse Chinese script type to a more complex short rod shape. When adding 0.8wt% Ba subsequently, the primary Mg₂Si further transforms from a regular tetragonal bulk shape to an irregular fine bulk shape with grooves and holes, and the eutectic Mg₂Si has an obvious refining effect and is diffusely distributed with the dot/short-line shape in the alloy matrix. The best metamorphic effect is obtained by adding 0.6wt% Er and 0.8wt% Ba. The mechanical properties of the Mg-2.5Si-4Zn alloy modified by Er-Ba composite modifier are significantly improved, with the tensile strength σ_b and elongation δ increasing to 168 MPa and 5.04%, respectively.