The invention discloses a composite nanometer waterproof and heat-insulating coating for buildings and a preparation method thereof, belonging to the technical field of coatings. The coating of the present invention comprises the following raw materials in parts by weight: 150-200 parts of emulsion, 60-80 parts of water-based polyurethane, 60-80 parts of filler, 60-80 parts of heat-insulating composite nanomaterial, 30-50 parts of silicone hydrophobic agent, film-forming 5-10 parts of auxiliary agent, 3-6 parts of defoamer, 3-6 parts of synergist. The present invention adds a large amount of heat-insulating composite nanomaterials, and through two-step modification of acid etching and hydrophobicity, the hydrophobicity and heat-insulation performance of the material is greatly improved. At the same time, the synergist added has strong adhesion ability and can Adsorbed on the surface of nano-materials and filler particles, it can be stably dispersed in the coating, greatly reducing the peeling and cracking of subsequent coatings, and prolonging the service life of the coating. The coating of the invention has good heat insulation effect, good waterproof performance, long service life and broad market application potential.