Silicon (Si)-containing aluminum (Al) alloys are highly valued in the fabrication of large-scale components with thin walls and complex geometries, such as automobile engine housings, gas turbine blades, and electrical equipment housings. These alloys are favored owing to their high fluidity, excellent filling ability, low risk of hot cracking, and excellent weldability. However, the broad solidification range of these alloys can lead to the formation of coarse primary Al dendrites and casting defects such as shrinkage porosity. To improve casting quality, inoculation is commonly carried out in practice. Numerous inoculants, such as Al-Ti, Al-B, Al-Ti-B, Al-Ti-C, and Al-Ti-B-C, have so far been developed. Among these, Al-Ti-B is widely adopted in industry owing to its high grain refinement efficiency. However, its efficiency decreases significantly when the Si content in Al alloys exceeds 5% (mass fraction), a phenomenon known as “Si poisoning”. To this end, an Al-Nb-B inoculant was developed to replace Al-Ti-B. Al-Nb-B demonstrates excellent grain refinement effect and effective resistance to Si poisoning, making it ideal for cast Al alloys with high Si contents. Typically, Al-Nb-B is fabricated using conventional casting methods with Al-Nb and Al-B intermetallic alloys as feedstocks. However, because these feedstocks have higher melting points than Al alloys, the reaction time required for the fabrication of Al-Nb-B is lengthy. This leads to the coarsening and sedimentation of nuclei in the molten Al, resulting in a non-uniform distribution in the as-cast inoculant, limiting its industrial application. To overcome this challenge, a fabrication method utilizing molten salt reactions has been proposed to homogenize the distribution of nuclei in Al-Nb-B inoculants. This approach not only improves the homogeneity of the nuclei but also reduces their average particle size from 10 μm to 1 μm. This is attributed to the relatively fast reaction rate between the molten salt and the liquid Al. As a result, the grain refinement efficiency improved significantly from 34% to 79%. Furthermore, plastic deformation aids in further homogenizing nucleus distribution. Hot extrusion is more effective than cold-rolling in this regard, showing the best results for enhancing grain refinement and antidegradation performance of the molten salt-based inoculant. The performance of this newly developed molten salt-based inoculant was verified during the fabrication of cast Al alloys ZL104 and ZL114A, which not only refines grain size by 79.2% and 78.5%, respectively but also significantly reduces casting defects. Consequently, the ductility and impact toughness of both ZL104 and ZL114A alloys improved simultaneously. This study provides a new approach to fabricating high-performance Al-Nb-B inoculants for cast Al alloys.