The invention relates to a block boron carbide-silicon carbide composite aerogel preparation method. The method includes: stirring nano boron carbide powder, resorcinol, formaldehyde, 3-aminopropyl triethyl silicane, absolute ethyl alcohol and deionized water according to a mole ratio to obtain faint yellow composite sol solution, and standing to obtain wet gel; subjecting the wet gel to solvent replacement, drying the wet gel according to a CO2 supercritical drying technique to obtain a boron carbide-silicon carbide precursor, and adopting a carbon thermal reduction technique in an inert atmosphere to obtain block boron carbide-silicon carbide composite aerogel. The block boron carbide-silicon carbide composite aerogel prepared according to the method has advantages of high specific surface, low density, high porosity, high temperature resistance and the like, a technical process is simple and easy in operation, and preparation of the block boron carbide-silicon carbide composite aerogel hopefully has a promising application prospect in fields of chemical equipment, high-temperature kilns and the like.