Abstract: The Al-xSi-4.5Cu-0.5Mg alloy (x=6%, 9% and 12%) alloy was prepared by extrusion casting. The effect of silicon content on the microstructure and mechanical properties was studied. The results show that content, the elongation of the alloy decreases obviously, and the tensile strength increases first and then declines. When the silicon content increases to 9wt%, the yield strength of the alloy is increased by 16.7% compared with that of the silicon content of 6wt%. Further increasing silicon content, the yield strength basically has no change.with the increase of silicon α-Al changes from columnar to equiaxed to columnar, the secondary dendrite spacing decreases, and the bright white second phase (Al₂Cu/AlSiCuMg)along the grain boundary increases first and then decreases.The number and depth of dimples in fracture morphology decrease, and the fracture mode changes from ductile fracture to brittle fracture.