A preparation method used for preparing a polyurethane hard foam insulation composite board includes the following steps: a fireproof polymer cement sheet is prefabricated; a polyurethane foaming machine manufacturing line is adopted to froth the PAPI and flame-retardant conjugate polyether into the polyurethane board and the polymer cement sheet is simultaneously compounded on two surfaces of the polyurethane board to prepare the multifunctional polyurethane hard foam insulation composite board. The preparation method of the fireproof polymer cement sheet comprises the following steps: after an adhesive and an inorganic powder are mixed according to the proportion of 1:1.5 to 2, the mixture is pressed with a non-woven cloth on the manufacturing line; the adhesive is prepared by the following materials according to the weight portions: 75 to 85 percent of polymer emulsion, 5 to 10 percent of flame retardant, 1 to 6 percent of film-forming additives, 5 to 10 percent of deionized water and 0 to 1.5 percent of thickener; the inorganic powder is prepared by the following materials according to the weight portions: 40 to 60 percent of stuffing, 10 to 25 percent of solid flame retardant and 25 to 40 percent of cement. The composite board prepared by the method integrates the fireproof, waterproof, high-intensity and insulation functions into a whole and can be used for the outer-wall waterproof, fireproof and insulation engineering of buildings.