Stubborn nonmetallic impurity boron is the most difficult to be removed by the metallurgical route for solar grade silicon production. To break through the restriction of separation of trace impurity, a novel approach by applying an electrical potential between the slag and the solvent phases to enhance silicon alloy refining using slag was developed. For silicon, Al-Si, and Sn-Si system, the removal efficiency of B increased with the augment of the external potential. When the external potential was increased to 6 V, the partition ratio of boron increased to 5.7, which was ten times than that without the external potential applied. Under the constant external potentials, the boron content decreased with increasing refining time. In addition, the synergistic removal mechanism of boron with electric field was investigated for the first time and the reactions both at the cathode and anode were explored, providing theoretical support for this new purification process.