Corundum-mullite composite castable uses fused mullite and corundum as a composite aggregate with a low-cement formulation. It has low thermal expansion and excellent thermal-shock stability, effectively resisting cracking and spalling caused by abrupt temperature changes, with stable volume at high temperature and high strength at medium/high temperatures. It is suitable for the roofs, walls and flue linings of various heat-treatment furnaces, heating furnaces and kilns, greatly extending the service life of batch kilns.