A hypoeutectic Al-Si matrix composite reinforced by 5wt% Mg2Si was prepared under different superheats and electromagnetic stirring (EMS), and the effects of pouring temperature (superheat) and EMS on the morphology and the grain size distribution of primary α-Al, (α-Al+Mg2Si) eutectic cell and eutectic Mg2Si were investigated. The results show that the low superheat + EMS process refines the grain structure of the primary α-Al and (α-Al+Mg2Si) eutectic cell, promotes the formation of more non-dendritic structure of the phases, as well as refines the eutectic Mg2Si and changes the nucleation mode of (α-Al+Mg2Si) binary eutectic from the nucleation on primary α-Al under conventional casting to the one on nucleant particles in the interdendritic liquid under low superheat + EMS.