A method for preparing aramid insulating paper, includes the operations of: providing aramid pulp slurry and aramid short fiber slurry at respective desired concentrations; blending, beating and fluffing the aramid pulp slurry and the aramid short fiber slurry in a given ratio to afford a paper pulp at a desired concentration; transiting the paper pulp to a papermaking machine fitted with a head box to form and make wet paper sheets; pressing to dewater the wet paper sheets and drying the dewatered paper sheets; and calendering the dried paper sheets at a high temperature to obtain the aramid insulating paper, wherein in operation iii), at least one ultrasonic generator is mounted on the head box to apply directional ultrasonic waves to the paper pulp flowing through the heat box. The aramid insulating paper prepared according to the method of the invention has enhanced strength, smoothness and uniformity.