An aramid-mica composite insulating nanopaper and a preparation method thereof are disclosed. A mixture of mica and urea is subjected to a solid-phase ball milling, to obtain nano mica modified with amino groups on the surface, which is then assembled with the aramid nanofiber by stepwise suction filtration according to the present disclosure, to form an ordered nanopaper, thereby obtaining an aramid-mica composite insulating nanopaper with ultra-high insulation performances.