To prepare thermal insulation refractory for high-temperature kilns with excellent thermal insulation and mechanical properties, porous spherical mullite-based castable was prepared with porous spherical mullite, bauxite powder, α-Al2O3 powder, silica powder, and Secar71 cement as main raw materials. The effect of bauxite powder content on mechanical properties, thermal conductivity, corrosion resistance, and thermal shock stability of porous spherical mullite-based castable was studied. The results show that changing the content of bauxite powder can improve the thermal insulation, thermal shock stability, and corrosion resistance of porous spherical mullite-based castable while maintaining high mechanical properties. As the content of bauxite powder increases, the mechanical properties of porous spherical mullite-based castable change little, but the thermal conductivity decreases slightly, the corrosion resistance is quite different, and the thermal shock stability increases first and then decreases. When the bauxite powder content is 28%(mass fraction), the mechanical properties, thermal shock stability, and corrosion resistance of porous spherical mullite-based castable are excellent, and the thermal conductivity at 1 000 ℃ is 0.905 W·m-1·K-1. The thermal conductivity of porous spherical mullite-based castable is lower than that of high-alumina castable for tundish and ladle permanent layer, therefore, it can be used as a substitute for high-alumina castable for tundish and ladle permanent layer, and its heat loss can be reduced.