Abstract <p>In this work, a novel alloy inoculant of Fe–V–Nb–C–RE(Ce) is successfully prepared by in-situ rapid solidification technology, aiming to enhance the mechanical properties of high-speed tool steel. The phase composition of this inoculant and its effect on the microstructure and mechanical properties of high-speed tool steel are systematically investigated. The results indicate that the prepared Fe–V–Nb–C–RE(Ce) inoculant features an amorphous-nanocrystalline composite structure, which can significantly refine the microstructure of high-speed tool steel and improve its mechanical properties. Specifically, after inoculation treatment, the grain size of as-cast high-speed tool steel is decreased by 32.9%, the dendrite spacing shortened by 39.6%, and the thickness of the reticulated carbides reduced by 26.4%. Consequently, the hardness and red hardness of the tempered high-speed tool steel is separately increased by 5% and 4%, the wear loss is decreased by 80%, and the impact energy is improved by 25%. These findings show that the Fe–V–Nb–C–RE(Ce) inoculant significantly enhances the overall mechanical properties of high-speed tool steel by refining grains and optimizing carbide distribution, providing theoretical support and technical guidance for its application in high-speed cutting tools.</p>