The invention provides a layered access-type extra high tension DC system differential protection method and device; the protection method comprises the following steps: determining whether the differential protection has started or not; if yes, determining whether the integral content of second harmonic in a three-phase different current is too low or not; if yes, carrying out differential treatment for the three-phase different current; determining whether differentiated each phase different current is saturated or not, if certain phase different current is not saturated, opening the differential protection of said phase; the concept that the integral content of second harmonic in the three-phase different current is too low refers to that the ratio between the second harmonic current ofthe maximum phase in the three-phase different current and a fundamental wave current is smaller than a first preset value. The method and device can fully consider the influences of the second harmonic current on the current, so the differential protection cannot fail when the second harmonic current content is too big.