The invention provides a prefabricated composite wallboard and a preparation method thereof. The prefabricated composite wallboard includes a first decorative protective layer, a reinforcing layer, asecond decorative protective layer, a vacuum insulation board layer and a structural layer which are sequentially attached; the first decorative protective layer and the second decorative protective layer are a mortar layer, the reinforcing layer is a high-strength fiber gridding cloth or a steel wire mesh sheet, and the structural layer is a reinforced concrete layer. According to the prefabricated composite wallboard and the preparation method thereof, the first decorative protective layer, the reinforcing layer, the second decorative protective layer, the vacuum insulation board layer and the structural layer are connected into a whole through connecting pieces, and the five layers of materials adopt strengths and avoid weaknesses, so that the integrated effects of lightness and thinness, high thermal insulation, fire protection, high strength, and high thermal performance are achieved. The prefabricated composite wallboard is more convenient to transport and install, the utilization of the building area is improved, and the problem that existing wallboards cannot integrate fire prevention, thermal insulation, lightness and thinness, and load bearing is solved.