The castables were prepared from spherical lightweight mullite aggregates, bauxite, SiO2 fine powder, α-Al2O3 fine powder and calcium aluminate cement. The performance changes of the castables were studied after heat treatment at 110 ℃ for 24 h, 1 100 ℃ for 3 h, 1 200 ℃ for 3 h and 1 300 ℃ for 3 h. The results illustrate that the cold modulus of rupture and hot modulus of rupture of the castable increase with the rising heat treatment temperature. The cold crushing strength shows a “V” change trend when the heat treatment temperature rises from 1 100 to 1 300 ℃. After the specimen is heat treated at 1 300 ℃, the thermal conductivity is 0.500 W·m^-1·K^-1, 0.586 W·m^-1·K^-1, 0.595 W·m^-1·K^-1 at 300, 600 and 900 ℃, respectively. The SEM analysis explains the change trends of these properties. The prepared lightweight mullite castables can be applied as the working lining of hot-blast furnaces.