Based on Maxwell equations, the Lorentz force in Al-Si alloy melt was calculated during the process of electromagnetic stirring using analytic approximation method. The effects of magnetic suspension force on the microstructure of Al-Si alloy were given under different electromagnetic stirring frequency. The surface layer for alloy melt (thickness is about skin depth) were under a larger magnetic suspension force in intermediate frequency electromagnetic stirring. As a result, it was found that Si phase tend to homogenize and aggregate along the direction of axis in the alloy.