The invention provides a fungus chaff composite plate. The components and content thereof (in percentage by weight) of the composite plate are: 48 to 54 percent of fungus chaff, 6 to 9 percent of straw, 6 to 9 percent of hard sawdust, ceramic fiber 5 percent of, 6 percent of talcum powder, 0.1 percent of ammonium dihydrogen phosphate, 5 percent of soybean flour, 0.7 percent of phosphoric acid, 0.3 percent of oxalic acid, 13.5 to 14.5 percent of urea-formaldehyde resin, 0.5 to 1.5 percent of polyvinyl acetate, 1 percent of polyvinyl alcohol, 0.7 percent of magnesium chloride, 0.1 percent of urea and 0.1 percent of hexamethylene tetramine. The preparation method comprises: 1, mixing materials; 2, mixing glue; 3, spreading; 4, applying glue; 5, performing hot pressing; 6, sanding; 7, spraying biological mothproofing agent; and 8, pressing film, pasting film or spraying paint. The composite plate has the advantages of wide source of raw materials, good waterproof property, good flame resistance, strong nail-holding power, mothproofing property and incapability of causing environmental pollution.