Al-Nb-B master alloy has been regarded as a promising grain refiner that can reduce grain size of hypoeutectic Al-Si casting alloys. However, its grain refinement performance remains to be improved. In this work, the grain refinement efficacy of Al-Nb-B master alloy is significantly enhanced by modifying the Nb/B ratio through thermodynamic calculation. An Al-Nb-B master alloy with optimum Nb/B ratio of 10:1 provides a fully equiaxed structure across the sections of the Al-10Si and commercial Al-9Si-0.08Ti alloys with an average grain size below 220 mu m. The phenomenon is attributed to the existence of NbAl3 and the higher number density of NbB2 at the Nb/B ratio of 10:1, which offers sufficient active nucleating sites to promote the formation of smaller grains. Moreover, the segregation behavior of Si atoms and interfacial energies after doping Si are investigated by first-principles calculations, and the results reveal that Si tends to segregate to the NbAl3/alpha-Al interface, whereas grain refining potency of NbAl3 for Al remains unchanged. This study has implications for strategic design of Al-Si cast alloy with fine and equiaxed grain structure inoculated by grain refiner.