Alumina aerogel thermal insulation composites were prepared by sol-gel and supercritical drying process, using aluminum sec-butoxide as precursor, and ceramic fiber as reinforcement. The microstructures were characterized by nitrogen adsorption experiments and scanning electron microscope (SEM), and the thermal insulation property was tested by hot plate method. The mechanism of thermal insulation of alumina aerogel composites was analyzed. The results show that, compared with silica aerogel, alumina aerogel can withstand even 1000℃ calcinations but still keeps the porous network. When alumina aerogel is introduced into the ceramic fiber, due to the excellent thermal resistance property of alumina aerogel, the thermal insulation property of composites is evidently improved, and the thermal conductivity of composites is 0.685 W/m·K at 1000℃.