The invention relates to a preparation method of a high-temperature-resistant insulating poly(m-phenylene isophthalamide) (PMIA)/montmorillonite nano-composite film. The method comprises the following steps: adding quaternary ammonium salt modified montmorillonite into a solvent, and carrying out ultrasonic dispersion; then, adding PMIA slurry for carrying out solution intercalation blending to obtain a mixed solution; after that, coating under the nitrogen atmosphere, carrying out vacuum stepped heating, and drying to form a film; cooling, demoulding, and carrying out vacuum drying again; then, cleaning for desalting, and carrying out vacuum drying to obtain the high-temperature-resistant insulating PMIA/montmorillonite nano-composite film. According to the method, the mixed solution obtained by using a solution intercalation blending method is taken as coating liquid, and the PMIA/montmorillonite nano-composite film is obtained by adopting a unique stepped drying technology and a simple desalting technology, so that the thermal stability, mechanical properties and insulating property of the PMIA film are further improved; the preparation method is convenient and practical, saves raw materials and cost, and can be easily in industrial production; the prepared high-temperature-resistant insulating PMIA/montmorillonite nano-composite film can be used for the field of electrical insulation and various high temperature protection fields.