In this study, a kind of microporous sintered alumina was analyzed and applied. The microporous sintered alumina has a lot of nano and sub-micron closed pores in the crystals, the average pore size of 140 nm, and the thermal conductivity of 4.864 at 800 ℃, 38% lower than that of common sintered alumina. The microporous sintered alumina was used as the aggregates in alumina magnesia castables for ladles to replace the common sintered alumina. The properties of the microporous alumina castable were measured, and the temperature field of the ladle outer wall was simulated and calculated. The results show that the refractoriness under load (T0.6) and the hot modulus of rupture (1 400 ℃, 0.5 h) of the microporous castable are the same as the common castable; the thermal conductivity at 800 ℃ decreases by 15%; the retention ratio of modulus of rupture increases by about 5% after five thermal shock cycles (1 100 ℃, water quenching); and the temperature of the ladle outer wall is reduced by 5-41 ℃.