The invention discloses a building external wall thermal insulation material and a preparation method thereof, and the building external wall thermal insulation material comprises the following components by weight: 70-90 parts of Portland cement, 8-15 parts of nano calcium carbonate, 20-30 parts of aminopropyltriethoxysilane, 5-15 parts of water glass, 1-5 parts of carbon fiber, 5-7 parts of polyvinyl alcohol, 6-11 parts of expanded perlite, 4-8 parts of mastra veneriformis shell powder, 2-4 parts of stannous chloride, 6-10 parts of polypropylene, 10-15 parts of phenolic resin, 8-14 parts of silicone modified epoxy resin, 6-12 parts of polystyrene resin, 4-10 parts of volcano ash, 8-12 parts of butyl acetate, 3-9 parts of magnesium oxide, 2-4 parts of magnesium chloride, 3-8 parts of calcite powder, 6-10 parts of graphite, 2-6 parts of mordenite powder, 2-5 parts of mica powder, 1-6 parts of aluminite powder, 2-4 parts of talc powder, 2-5 parts of bauxite, 1-3 parts of vanadium carbide, 1-5 parts of molybdenum disulfide, 1-3 parts of a foam stabilizer, 1-2 parts of a foaming agent and 100-260 parts of water. The building external wall thermal insulation material can save the amount of cement, reduces the construction cost, and reduces the environmental damage, and the compressive strength and the fireproofing thermal insulation performance of the material are improved.