The microstructure of A357 alloy modified by yttrium (Y) and its modification mechanism was investigated. The results show that under a sand casting condition, there was small or even no solution strengthening effect of Y on the A357 alloy. Y did not change the morphology of eutectic silicon but made it shorter, and Y increased the nucleation cores during its nucleation processes rather than promoting the formation of twinning during the growth processes. The solidification mode of many eutectic silicon phases changed from nucleating on the dendrites to heterogeneous nucleation of eutectic grains in the interdendritic liquid. The number and area of eutectic silicon clusters increased with increasing Y. A bright Al₂Si₂Y phase appeared even though Y was very low.