To control the morphology and size of the primary and eutectic Mg2Si phases in in situ Mg2Si/Al-Si composite and achieve a feasible and reliable technique to produce appropriate feedstock for the thixo-casting and rheo-casting of this type of material,three Al-Si matrix composites reinforced by 5wt.%,9wt.%and 17wt.%Mg2Si with hypoeutectic,eutectic and hypereutectic compositions were prepared by the low superheat pouring(LSP)process.The effects of the pouring temperature(superheat)on the morphology and size distribution of primary phases(primaryα-Al and Mg2Si),binary(α-Al+Mg2Si)eutectic cell and eutectic Mg2Si were investigated.The experimental results show that low pouring temperature(superheat)not only refines the grain structure of the primaryα-Al and binary(α-Al+Mg2Si)eutectic cell in three composites and promotes the formation of more nondendritic structural semi-solid metal(SSM)slurry of these phases;but also refines the primary and eutectic Mg2Si phases,which seems to be attributed to the creation of an ideal condition for the nucleation and the acquisition of a high survival of nuclei caused by the LSP process. To control the morphology and size of the primary and eutectic Mg2Si phases in situ Mg2Si / Al-Si composite and achieve a feasible and reliable technique to produce appropriate feedstock for the thixo-casting and rheo-casting of this type of material, three Al -Si matrix composites reinforced by 5 wt.%, 9 wt.% And 17 wt.% Mg2Si with hypoeutectic, eutectic and hypereutectic compositions were prepared by the low superheat pouring (LSP) process. The effects of the pouring temperature (superheat) on the morphology and size distribution of primary phases (primary α-Al and Mg2Si), binary (α-Al + Mg2Si) eutectic cell and eutectic Mg2Si were investigated. The experimental results show that low pouring temperature (superheat) not only refines the grain structure of the primary α- Al and binary (α-Al + Mg2Si) eutectic cell in three composites and promotes the formation of more nondendritic structural semi-solid metal (SSM) slurry of these phases; but also refines the primary and eutectic Mg2Si phases, which seems to be attr ibuted to the creation of an ideal condition for the nucleation and the acquisition of a high survival of nuclei caused by the LSP process.