The present invention relates to an extra-high voltage transformer differential protection method and device. The method comprises the steps of: calculating an extra-high voltage transformer differential protection current, and performing differential protection starting distinguishing; if the differential protection starting distinguishing is met, determining whether second harmonics comprehensive content in a three-phase differential current is lower than a first second harmonics comprehensive content set value or not; if not, determining whether a ratio between the three-phase differentialcurrent and a brake current is larger than a ratio set value or not; and if the ratio between the three-phase differential current and the brake current is larger than the ratio set value, opening thephase differential protection. When the maximal phase second harmonic current content is high and a certain phase differential current is larger than the brake current, the phase differential protection is opened so as to solve the problem of the protection device delay motion caused by fault magnetizing inrush current in a generation area when the extra-high voltage transformer is normally operated and improve the reliability of the protection device.