The invention discloses composite heat preservation light autoclaved aerated concrete. The invention discloses a preparation method of the composite thermal-insulation lightweight autoclaved aerated concrete. The preparation method comprises the following steps: mixing and ball-milling fluorite mine tailings, quartz sand, building solid waste and water to obtain mortar; adding lime, cement, composite sepiolite and aluminum powder into the mortar, uniformly stirring, pouring into a mold, pouring, and carrying out gas forming to form a green body; and demolding the green body, cutting, and steaming to obtain the composite heat-preservation light autoclaved aerated concrete. The composite sepiolite is prepared by the following process: feeding sepiolite powder into a sulfuric acid solution, stirring for 1-2 hours at 50-60 DEG C, filtering, washing to be neutral, drying, then adding sodium alginate, microcrystalline cellulose and water, fully and uniformly mixing, carrying out hydrothermalcarbonization for 2-4 hours at 160-180 DEG C, filtering, washing, drying and crushing to obtain the composite sepiolite. The autoclaved aerated concrete is low in density, low in heat conductivity coefficient and good in sound insulation and compressive strength.