The invention belongs to the technical field of purifying polycrystalline silicon by physical metallurgy technology. A method for purifying polycrystalline silicon by directional solidification and filter slag melting comprises the following steps: heating slag former in a melting crucible, keeping the slag former in a liquid state, simultaneously, melting polycrystalline silicon material with high boron and high metal in the other small crucible; introducing and dispersing polycrystalline silicon melt into the liquid slag former, conducting melting reaction to remove impurity boron, and when the melting crucible is fully filled with liquid, stopping adding polycrystalline melt, heating so as to lead the mixture in the melting crucible to be in a liquid state, conducting directional solidification on the mixture after melting, cutting polycrystalline silicon and waste residue with higher impurity content at the top of a silicon ingot, thus obtaining the polycrystalline silicon ingot with a lower boron and metal impurity content. The impurity boron and metal in the polycrystalline silicon are removed by filter slag melting and directional solidification, therefore, the purity of the polycrystalline silicon is improved effectively, the using requirement of solar silicon is achieved, the purifying effect is good, the technique is stable, the process is simple, the energy is saved, the cost is low and the production efficiency is high.