Numerous studies have searched for methods of Si refining for the production of photovoltaic cells to alleviate the global shortage of solar-grade Si. An alloy refining progress with a Si-Al-Sn melt was developed in this paper. To elucidate the relationship between Sn and B, the interaction parameter between them at 1173K was determined as 2506 +/- 143. This result indicated the strongly repulsive force between Sn and B. Thermodynamic analysis and experiments showed that the purification efficiency of Si-Al-Sn alloy weakened with increased Sn content in the melt.