In order to improve mechanical properties of Mg₂Si/Al composites, Eu element was added to modify Mg₂Si morphology, and T6 heat treatment was used to control Mg₂Si morphology and precipitate strengthening phase. Microstructure and morphology of Mg₂Si were observed by synchrotron X-ray tomography and TEM, and first-principles calculations were also performed to testify the effect of Eu modification. Results show that the size of primary Mg₂Si particles decreases and eutectic Mg₂Si phase transforms from lamellar to fibrous by Eu addition. After heat treatment, sharp angles of primary Mg₂Si particles passivate, and eutectic Mg₂Si dissolves and appears to be short dot-like. Meanwhile, nano-sized β" phase precipitates in the matrix. For morphology of Mg₂Si with Eu modification, TEM results show that Eu impedes the growing of Mg₂Si, which is verified by first-principles calculations that Eu atom preferentially adsorbs on Mg₂Si {100} facet. The adsorption and suppression growing of Mg₂Si transform the morphology of Mg₂Si and thus improve the elongation. UTS and elongation of the heat-treated Eu modified composites are 281 MPa and 8.4%, which improved 81% and 200% compared to the as-cast Al-15%Mg₂Si composite. The strengthening mechanism mainly results from precipitation strengthening of nano-sized β" precipitates in the heat-treated composite.