The invention provides inorganic geopolymer foamed concrete, a composite wallboard prepared by the inorganic geopolymer foamed concrete, and a preparation method thereof. The inorganic geopolymer foamed concrete comprises the following components in parts by weight: 400 to 500 parts of inorganic geopolymer cementing material and 0.5 to 10 parts of foaming agent, wherein the inorganic polymer cementing material comprises 2 to 20 parts by weight of active silica-alumina material, 4 to 30 parts by weight of alkali salt aqueous solution, 1 to 20 parts of inorganic filler, and 0.05 to 3 parts of setting agent. The lightweight and high-strength inorganic geopolymer foamed concrete sandwich composite thermal insulation wallboard can be prepared from the inorganic geopolymer foamed concrete provided by the invention. The preparation method provided by the invention has the advantages of simple process and high production efficiency, and the thermal insulation and decoration integrated construction of exterior walls of buildings can be realized.