The primary silicon is still rough in hypereuctic Al-Si alloys with the P modification. Aimed at this, the melt stirring is proceeded after the Al-15%Si, Al-18%Si and Al-20%Si alloys were modified by the Cu-9%P alloy, and the influences of the stirring parameters on the modification effect were studied. The mechanism that the melt stirring can improve the modification effect was analyzed. The results show that the melt stirring can obviously enhance the modification effect of hypereuctic Al-Si alloys modified the Cu-P alloy, and the primary silicon can refine the maximum 85%. Within the optimum stirring time, the stronger the stirring is and the higher the silicon content is, the better the modification effect is. The melt stirring compel the AlP in the region of rich P to stop the growth. The faster the velocity of the melt flow is, the stronger the scour of AlP in rich P region is. The easier it is to obtain the fine AlP. The melt stirring can also increase the quantity of the primary α in the microstructure. The typical eutectic structure basically disappears if the stirring parameters are suitable. The microstructure of the tiny primary silicon evenly distributed in the continuous α matrix is obtained.