To obtain high-performance Al-Si-based cast alloys, refinement and modification of Si phases are required. An Al-12Si/ZnS powder inoculant was designed and fabricated using a chemical bath deposition method. The efficiency of the inoculant for modifying the eutectic Si phase in as-cast Al-12Si alloy was studied. Results show that Al-12Si/ZnS powder can significantly refine the eutectic Si in Al-Si cast alloys. The best refinement effect for eutectic Si was achieved with 17.5% Al-12Si/ZnS powder. Coarse long needle-shaped eutectic Si with a length of 18 μm was modified into approximately spherical shape with a diameter of 7 μm, which was evenly distributed throughout the alloy. The E2EM model calculation indicates that the inter-plane misfit (Fp) and inter-atomic spacing misfit (Fr) between ZnS and Si are all less than 0.5%, which confirms that ZnS is a potential nucleation site for Si phases. The hardness, tensile strength, and elongation of Al-12Si alloys modified with 17.5% Al-12Si/ZnS powder increase 6.3%, 16.19% and 55.35%, respectively, compared to the unmodified Al-12Si alloy. The fracture behavior of the alloy with 17.5% Al-12Si/ZnS powder was dominated by transgranular fracture supplemented by intergranular fracture.