The invention relates to an extra-high voltage regulating transformer differential protection method, which comprises the following steps: determining a differential current value Id of a regulating transformer according to primary side current of the extra-high voltage regulating transformer; if the differential current value Id is smaller than a differential current setting value, returning the differential protection, or otherwise, entering the next step; detecting whether secondary harmonic content P2nd in the Id is smaller than a harmonic content constant value K2nd; if so, judging that the voltage regulating transformer has an internal fault during operating, and carrying out differential protection immediately, or otherwise, entering the next step; calculating second harmonic change rate C (k), and detecting whether the C (k) meets operation criterion; and if so, judging that the voltage regulating transformer has the internal fault during core saturation due to DC magnetic bias, and the secondary harmonic content changes abruptly, or otherwise, carrying out calculating and comparison of the next sampling point. The technical scheme can prevent the case that when a fault occurs under the DC magnetic bias condition, extra-high voltage regulating transformer protection conventional criterion refuses to act due to second harmonic locking, and the extra-high voltage regulating transformer is not influenced by the DC magnetic bias.