Si particles reinforced Al matrix composites (Sip/Al) have been widely used in the electronic packaging filed, and the higher mechanical and themo-physical properties are required for their application. In the present work, the effect of Si content (from 0.28 to 6.95 wt. %) in the matrix alloy on the microstructure, mechanical and thermo-physical properties of Sip/Al-Si-Mg composites have been investigated. Sip/Al-Si-Mg composites with 55 vol. % Si particles were prepared by pressure infiltration method. It has been found that the increment of Si content led to the improvement of relative density of composites. Moreover, the bending strength and thermal conductivity of Sip/Al-Si-Mg composites reached its maximum at Si content of 1.98 wt.%, which were 238.8MPa and 159.5W/(m∙K), respectively. Furthermore, the coefficient of thermal expansion (CTE) of Sip/Al-Si-Mg composites decreased with the increase of Si content. The mechanism of Si content on the microstructure of performance of Sip/Al-Si-Mg composites has been discussed.