The invention belongs to the technical field of heat insulating material preparation, and particularly relates to a preparation method of a corundum/mullite light-weight heat insulating material, which comprises the following steps: by using ceramic powder, silica sol, deionized water, ammonia water and dryness-control chemical additive as initial raw materials, carrying out ball milling to prepare a ceramic slurry; adding a foaming agent into the ceramic slurry, and foaming in a mechanical stirring mode to obtain a foam slurry; adding an NH4Cl solution into the foam slurry while stirring, injecting the uniformly stirred foam slurry into a mold, and quickly curing to form the foam slurry; and drying the demolded blank, and firing to obtain the corundum/mullite light-weight heat insulating material. The invention is characterized in that the gelation reaction of the silica sol is utilized to implement curing and formation of the foam slurry; meanwhile, the volume expansion generated by mullitization in the firing process is utilized to avoid sample cracking due to excessive sintering shrinkage; and in addition, the raw materials are nontoxic and cheap, and the preparation process is simple to control and suitable for industrial production.