In this paper, titania–alumina composite aerogels containing different contents of potassium titanate were synthesized by sol–gel process and supercritical ethanol drying with potassium titanate as the precursor. Moreover, the effects of potassium titanate addition on the structures and properties of composite aerogels were investigated. The investigation results showed that contents of potassium titanate significantly affected the structures and morphologies of composite aerogels. When potassium titanate content was low, potassium titanate could be decomposed into titania and potassium ions could be removed by ion exchange. Fibriform alumina and sheet titania nanoparticles were aggregated to form flower-like or rod-like clusters. However, the high content of potassium titanate resulted in the residues of unreactive potassium titanate. Meanwhile, excessive titania easily converted the cluster shape into the spherical shape, and titania particles grew and stretched out of the spherical clusters to form titania sheets, thus displaying the cactus-like morphology. In composite aerogels, alumina nanoparticles could hinder the sintering of titania particles and retard the anatase-to-rutile phase transformation until the temperature reached 1000 °C, thereby improving the heat resistance of anatase titania. The composite aerogels showed the high surface area and low thermal conductivity and might be applied as thermal insulation materials or catalysis supports.