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.