The invention discloses a preparation method of a thermal-insulation composite material for a building. The preparation method comprises steps as follows: deionized water, carbon black, cadmium sulfoselenide, copper oxide, rutile titanium dioxide, aluminium silicate fibers, ceramic fibers, glass fiber balls, polyvinyl acetate, polyethylene oxide, polyvinyl alcohol, phthalocyanine blue, ultramarine blue, methyl trichlorosilane and toluene. The preparation method is simple, the thermal insulation effect is good, the cost is low, and the material is fast to dry, is not hydrolyzed or powdered after being soaked in water for 4-8 h, can be resistant to the high temperature of 900 DEG C without falling off and resistant to acid and alkali and has excellent anti-humidity performance; a product after film formation is high in strength and good in corrosion resistance and is non-corrosive to walls, metals, wood and plastics, the adhesive force is in the level 1, the impact strength is 60-100 J, and the hardness is 5 H; the material is unchanged under distilled water at the temperature of 50 DEG C for 4-8 d, the flame-retardant effect is good, the service life is long, the toughness is 1 mm, and the glossiness is good; the material is free of bubbles and cracks after being soaked in a sodium chloride solution for 3-7 d, the raw materials are widely sourced, and the material can be extensively produced and continuously replace existing materials.