The invention discloses a composite heat-insulating wallboard and a preparation method thereof and relates to a heat-insulating wallboard. The heat-insulating wallboard is provided with two layers of calcium silicate plates, wherein a heat-insulating core material is clamped between the two layers of calcium silicate plates and is prepared from cement, coal ash, gypsum, expandable polystyrene (EPS), Maleamic Acid-Isobutyl Polyhedral Oligomeric Silsesquioxane (POSS), emulsion powder, a water reducing agent, a foaming agent and an early strength agent. Dry POSS powder is dissolved in normal hexane and then the solution is sprayed onto the surface of EPS particles to obtain substance A; the cement, the coal ash, the gypsum and the emulsion powder are drily stirred, water is added into the mixture, the mixture is stirred into paste, the substance A is added into the paste to form a slurry B; the foaming agent solution is stirred through a stirrer to form foam C; the slurry B and the foam C are mixed to obtain a heat-insulating core material slurry; the surfaces of the calcium silicate plates are brushed and washed, the calcium silicate plates are placed on the two sides of a fixed mold, the heat-insulating core material slurry is poured into the middle of the two calcium silicate plates, and the mold is vibrated to mold the slurry; and the mold is de-molded, and after watering to maintain, the wallboard is obtained.