The silica-alumina composite aerogels were prepared by a sol-gel method combined with the ethanol supercritical drying technique using aluminum tri-sec butoxide(ASB) and tetraethylorthosilicate (TEOS) as the precursor and nitric acid as catalyzer respectively.Si to Al molar ratio is 3∶1 according to the composition of mullite.The effects of the catalysts amount on the gelation time of aerogels were studied.The aerogels samples were heat-treated at different temperatures of 600,800 and 1000℃.The structure and thermal capability of the composite aerogels are characterizated by BET, XRD, SEM and Hotdisk etc.The silica-alumina composite aerogels were exhibited excellent thermal insulating properties.The elastic modulus of the composite aerogels was up to 5.3MPa after heat treatment at 1000℃.The thermal conductivity the composite aerogels were increased from 0.035 to 0.058 W/m.K with the rise of temperature.