The invention relates to a nanometer heat-insulation exterior coating and a preparation method thereof. The coating comprises the following ingredients (by weight): 25-40 parts of a resin synthetic emulsion, 30-45 parts of powder, 3-5 parts of an auxiliary agent and 20-30 parts of water. The preparation method comprises the following steps: firstly adding water into a stirring tank and stirring at low speed, adding part of the auxiliary agent and uniformly mixing, adding powder, raising stirring speed of the stirring tank and dispersing, reducing stirring speed of the stirring tank, successively adding the resin mixed emulsion and the other part of the auxiliary agent, and adjusting to required viscosity by adding a thickening agent so as to prepare the nanometer heat-insulation exterior coating. In comparison with the prior art, the nanometer heat-insulation exterior coating has excellent heat insulation property, water resistance, stain resistance, etc.