Al-Si alloy purification method is an up-and-coming method for the preparation of solar-grade polysilicon raw materials because of its low production cost, high removal efficiency and single by-product. In this process, Al as a solvent will inevitably contaminate Si, and how to reduce the content of Al in primary silicon is one of the urgent problems to be solved. In this paper, a small amount of Cu was added to Al-50%Si alloy, the effect of Cu on the thermomechanical properties of the alloy solution was analyzed, and the inhibitory effect of Cu on Al contamination in combination with the existing form of Cu was explored. The results show that the activity coefficient of Al in the alloy reduces to 0.714 8 with 10% (mass fraction) Cu addition. The content of Al in the primary silicon reduces from 250.960 mg/kg to 181.637 mg/kg, which is 27.62% less than that without the addition of Cu. Meanwhile, the residual Cu in primary silicon is only 12.6 mg/kg, which is lower than the solid solution of Cu in Si. The introduction of Cu into the Al-Si alloy does not cause secondary contamination of the primary silicon. Therefore, using Al-Si-Cu ternary alloy system for purification to prepare solar-grade polysilicon can effectively suppress Al contamination of primary silicon.