The present invention discloses a preparation method for preparing the external insulation material of a high-voltage outdoor composite insulator and belongs to the technical field of electric materials. According to the technical scheme of the invention, rice husks are mixed up with fly ashes. Through the fermentation of the supernatant of a biogas broth, the organic matters of rice husks are consumed and then metal elements therein are dissolved. After the filtering and drying process, an obtained filter cake is calcined at a high temperature, so that organic matters are sufficiently removed. After that, resulting micro nano powders are made into a modified sol, and the modified sol is subjected to reaction with a self-prepared acrylate emulsion. In this way, a composite emulsion is obtained. The composite emulsion is coated onto the surface of an insulator through the spin-coating process. After the drying process, the external insulation material of the high-voltage outdoor composite insulator is prepared. According to the technical scheme of the invention, a micro-nano structure similar to a lotus-leaf-like surface is formed on surface of the insulator by utilizing micro-nano particles contained in the composite emulsion. Therefore, the super-hydrophobic effect is realized, and the ultraviolet light is effectively absorbed. Meanwhile, the anti-aging property of the insulator is increased and the service life of the outdoor insulator is prolonged.