The effects of different contents of rare earth La on the microstructure and properties of cast Al-Si binary alloys were investigated. The results indicate that the addition of a small amount of La can improve the aggregation of eutectic silicon, weaken the cleavage of the matrix, and refine the nearly nondendritic microstructure of the aluminium matrix when the La content is 0.15 wt. %. Moreover, the size of eutectic silicon is significantly reduced. As the La content increases, the yield strength and elongation of the Al-Si alloy initially increase and then stabilize, while the electrical conductivity gradually increases. The refinement of eutectic silicon reduces the hindrance of Si to electron scattering, thereby enhancing the electrical conductivity of the alloy. Overall, when 0.15 wt. % La is added, the cast Al-Si alloy has the best overall microstructure, mechanical properties and conductivity.