Al-xMg₂Si alloy was modified with 0.5 wt.% Ca-Sb. With the decrease of Mg/Si content from 18 wt% to 14 wt%, the grain size of primary Mg₂Si in the unmodified alloy was refined from ~ 123 mm to ~ 15 mm, and the grain size of primary Mg₂Si in the unmodified alloy decreased from ~ 123 mm to ~ 12 mm When the content of Mg/Si in the alloy is reduced to 10 wt%, the microstructure of the alloy is composed of primaryα- Al + eutectic Mg₂Si. In Al-xMg₂Si alloy modified by 0.5 wt.% CA sb, dendrite like primary Mg₂Si disappears and transforms into fine polygon due to the effect of modifier. With the decrease of Mg/Si content from 18 to 12 wt.%, the grain size of primary Mg₂Si decreases from ~ 15 mm to ~ 5 mm, forming ultrafine primary Mg₂Si. The mechanical properties of the alloy were tested, and it was found that ultrafine primary Mg₂Si had a great effect on improving the mechanical properties of the alloy. The optimum alloy is Al-12Mg₂Si modified by 0.5 wt.% Ca-Sb.