The invention relates to a preparation method of a micro/nano composite fiber electrical insulation paper, and belongs to the technical field of electrical insulation paper. The preparation method comprises the following steps: individually soaking a dry pulp board of softwood pulp and a dry pulp board of other micro fiber pulp in deionized water for 12 hours, after soaking, tearing the pulp boards into pieces with a size of 25 mm*25 mm; then decomposing and beating the processed softwood pulp board and other micro fiber pulp board by a beating machine; carrying out an ultrasonic dispersion treatment, mixing a proper amount of softwood pulp, other micro fibers, and nano fibers, and processing the mixture for 5 to 20 minutes by a stirring device. A mixing beating mode can also be adopted. Namely, preprocessed softwood pulp board and other micro fiber pulp board are directly mixed according to a certain ratio, then the mixture is decomposed and beaten by a beating machine, and the Schopper-Riegler beating degree is controlled in a range of 10 to 60 degrees SR. The process is simple, the operation is easy, and the tensile strength and breakdown field strength of prepared electrical insulation paper are both improved effectively.