To enhance the understanding of impurities B and P removal in the solidification purification process of the Al-30Si alloy, series of experiments were carried out to study the roles of AIB(2) and AlP in the nucleation of primary Si and the effect of electromagnetic stirring on the segregation behaviors of B and P. The results showed that the formed AIB(2) phase does not act as the nucleation site for the primary Si and tends to distribute in the eutectic Al-Si, which would not pollute the primary Si. The P content (< 68 ppmw) in metallurgical silicon is not sufficient to form AlP phase before the nucleation of primary Si, and thus the AlP phase can be removed effectively as it would be pushed to the front of the solid-liquid interface during the primary Si growth. It was also found electromagnetic stirring could effectively enhance the segregation behaviors of B and P during the growth of the primary Si, where the B and P contents in the primary Si were decreased with the increase of the magnetic field intensity. The effective segregation coefficients of B and P were reduced from 0.41 to 0.23 and 0.47 to 0.3, respectively, under 25 mT magnetic field compared that without electromagnetic stirring.