The invention discloses a preparation method of a coal ash modified nanometer aluminum oxide / polyimide composite film. The preparation method can be used for preparing the nanometer aluminum oxide hybrid polyimide film by utilizing nanometer aluminum oxide sol, so that corona-resistant characteristics and dielectric performances of the polyimide film can be remarkably improved, and the corona-resistant service life of the nanometer aluminum oxide hybrid polyimide film is 8-9 times that of the pure polyimide film. The preparation method further can be used for improving and overcoming the defects of the pure polyimide film, remarkably improving the mechanical strength, the hardness, the abrasion performance and thermal decomposition temperature of the pure polyimide film, so that the comprehensive performances of the pure polyimide film are improved. Moreover, the novel polyimide nanometer composite film prepared by the preparation method disclosed by the invention has the characteristics of being high temperature resistance, good in mechanical performances, excellent in electrical performances, easy to machine and form, low in cost and the like, so that the development requirements of high and new technologies including the aerospace technology, the micro-electronics technology, the electrical technology, the chemical technology, the energy technology and the like can be completely satisfied.