This invention relates to a relay protection method in a power system and discloses a method for realizing circuit differential protection based on the long-line equation computation in a high voltage transmission circuit, which converts the voltage and current tested at the protection mounted place to those of a reference point K in the protection sphere by a method of vector computing long-line equation, in which, the voltage Um and the current Im of end m are voltage UKm and current IKm converted to point K, the current In is converted to point k as voltage Ukn and cutrrent Ikn, if there are parallel reactors, it's needed to deduct the current on it to get I'km and I'kn, then Ikm, Ikn or I'km applied at the reference point and the I'kn is computed in terms of multiple differential formula to judge if there is fault in the region.