The invention discloses a composite fireproof insulation board and a preparation method thereof. The composite fireproof insulation board can be selected from a two-layer structure including an insulation layer and an inorganic fireproof insulation layer or a three-layer structure including an insulation layer, an inorganic fireproof insulation layer and a mortar protection layer; the insulation layer is selected from one of a polyurethane plate, an extruded plate, an expanded polystyrene plate and a thermosetting polystyrene foamed plastic; the inorganic fireproof insulation layer is prepared by selecting 80 parts by mass of magnesium oxide, 60 parts by mass of magnesium chloride, 40 parts by mass of water, 80 parts by mass of a mineral powder, 1 part by mass of a modifier, 8 parts by mass of a hydrophobic agent, 2 parts by mass of a hydrophobic powder, 8-10 parts by mass of expanded polystyrene particles, and 25 parts by mass of a foaming agent and water; the mortar protection layer is prepared from 350 parts by mass of cement, 650 parts by mass of quartz sand or washed-out sand, 1 part by mass of hydroxypropyl methyl cellulose, 1 part by mass of cellulose ether, 20 parts by mass of a rubber powder, and 7 parts by mass of a hydrophobic agent. The composite fireproof insulation layer provided by the invention has the advantages of fire prevention, thermal insulation, environmental friendliness and the like.