The invention discloses a plastered fireproof stone composite reinforcing insulation board for a building and a preparation method of the insulation board.A plastering mortar layer is composited on the base surface of a foamed phenolic resin material and prepared from, by weight, 250-500 kg of quartz sand, 150-250 kg of Portland cement, 100-150 kg of fly ash, 50-100 kg of wollastonite, 5-25 kg of inorganic pigments and 20-50 kg of an additive.According to the insulation board and the preparation method thereof, the plastering mortar is composited on the phenolic resin material, therefore, the strength is improved while the thermal insulation property of the building is improved, and the insulation board is not prone to damage and crack; stone is composited with the insulation board, therefore, the decorative effect of the building is greatly improved, the advantages of being not prone to fall off, easy to construct and the like are achieved, and the cost input is low; the preparation technology is simple and environmentally friendly, industrialized production is easy to achieve, and the good social and economic benefits are achieved.