Abstract: With the increasing demand for improvements in the temperature capability of aero-engines, there is an urgent need to develop new-generation turbine blade materials. Compared with Ni-based superalloys that have a lower melting point (~1300 ℃), the higher melting point (>1750 ℃), lower mass density (6.6–7.2 g·cm^–3), and high-temperature strength of the Nb–Si based alloys make them one of the most promising of the new-generation high-temperature structural materials. A directional solidification process can further enhance the performance of Nb–Si based alloys and lay a foundation for replacing the Ni-based single-crystal superalloys in service at higher temperatures. Accurately determining the thermal property parameters of Nb–Si based alloys and their interfacial heat transfer behavior during solidification is the key to their numerical simulation, which could accelerate the development of Nb–Si based alloys. As yet, however, there has been no research reported in relation to this issue. In this study, we used the directional solidification process of Nb–Si based alloys as the research object and the experimental testing and reverse methods to determine the thermal properties of Nb–Si based alloys and their shells as well as the boundary conditions of the heat transfer coefficient at the interface during the solidification process. To simulate the temperature field of the solidification process of Nb–Si based alloys at different drawing rates, we used ProCAST software. The results reveal that as the withdrawal rate increased from 5 to 10 mm·min⁻¹, the distance between the solid/liquid interface and the surface of the liquid metal tin decreased from 12.1 to 8.2 mm, and the average width of the mushy zone gradually narrowed from 11.5 mm to 10.4 mm. The discrepancy in the spacing of the primary dendrites between the numerical simulation and the actual experimental results at a withdrawal rate of 5 mm·min⁻¹ was within 6%, which verifies the correctness of the temperature-field simulation results. These results provide reference for the determination of the directional solidification casting parameters of turbine blades made of Nb–Si based alloys.