A new analysis and filtering method for the ultrahigh order harmonic from the alternating current (AC)direct current (DC)AC main drive loads was proposed. The method was based on the practice and the new notation of the ARU（active rectifier unit） main transformer, and the specification of the integrated gate commutated thrusters (IGCT) and injection enhanced gate transistors (IEGT). By analyzing the features of the ultrahigh order harmonic, the harmfulness to electrical equipment and manufacturing as well as the necessity to filter ultrahigh order harmonic, indicates it is necessary to set up a higher frequency (HF)type filter. This research serves as a reference for designing and manufacturing the HFtype filter in the power supply of rolling mills.