The invention discloses an organic-inorganic composite nanometer heat-insulating fire-retardant coating and a preparation method thereof. The organic-inorganic composite nanometer heat-insulating fire-retardant coating is prepared by mixing organic film-forming substance acrylic ester emulsions or styrene-acrylate emulsions, epoxide resin, organic intumescent fire-retardant system pentaerythritol, melamine and ammonium polyphosphate with inorganic fire retardant nanometer aluminum hydroxide, nanometer magnesium hydroxide, filler nanometer titanium dioxide, ceramic hollow microspheres, glass hollow microspheres and a part of auxiliary agents. The organic-inorganic composite nanometer heat-insulating fire-retardant coating disclosed by the invention not only has excellent facing property and fire-retardant property, but also achieves reflection effect on solar radiation and certain heat-insulating and heat-preserving property on a substrate, also has good adhesivity, mechanical property and hydrophobic effect, can be formed through spray coating or brush coating before being solidified, and is a novel water-based multifunctional fire-retardant material. The organic-inorganic composite nanometer heat-insulating fire-retardant coating disclosed by the invention can not only be used as a general building wall body fire-retardant coating, but also be suitable for the fire prevention of building and ornament materials, heat-insulating materials, steel structure supports, beam columns, metal roofs and the like.