The　as-casted　Al-Si-Mg　alloy　was　treated　by　solution　and　aging　process　of　545°C　/10h/water　cooling　plus　175°C/6h　/air　cooling.　The　effect　of　heat　treatment　on　the　microstructure　and　mechanical　property　of　Al-Si-Mg　was　investigated　by　metallographic　analysis,　scanning　electron　microscopy,　energy　dispersion　spectrum　analysis　and　mechanical　testing.　The　experimental　results　showed　that　the　alloy　had　the　ultimate　tensile　strength　(UTS)　of　317MPa　and　the　elongation　of　2%,　and　suitable　for　squeezing　cast.　During　solution　treatment,　the　plate-like　eutectic　Si　particles　became　small　granular　or　short　bacilliform　morphology,　and　the　non-uniformly　distributed　eutectic　phase　was　eliminated　substantially.　In　addition,　Si　particles　distributed　uniformly　and　finely　in　the　matrix.　The　tensile　strength　of　as-casted　alloy　was　180　MPa,　while　it　was　up　to　317　MPa　after　solution　and　aging　treatment　process,　and　the　elongation　increased　from　2%　to　3%,　which　is　consistent　with　the　microstructure.　Fracture　surface　analysis　showed　that　fracture　mode　of　the　alloy　transformed　from　brittle　fracture　into　co-existence　of　ductile　fracture　and　brittle　fracture　during　T6　treatment.　©　2016　Trans　Tech　Publications,　Switzerland.