The invention discloses a false synchronization differential protection sampling data rollback method and device of a half-wavelength power transmission line. The false synchronization differential protection sampling data rollback method comprises the steps of comparing passage delay time Tx with a power frequency period T; performing false synchronization differential impedance calculation on acurrent sampling point of sampling data rolled back to (Tx-T) moment of a side and a received sampling point of an opposite side if Tx is larger than T; performing false synchronization differential impedance calculation on the current sampling point of the side and the received sampling point of data rolled back to (T-Tx) moment of the opposite side if Tx is smaller than T; and unnecessarily performing rollback on the data if Tx is equal to T. By the false synchronization differential protection sampling data rollback method, the problem of longitudinal passage delay of false synchronizationdifferential protection in the half-wavelength power transmission line is solved in an emphasized way, false synchronization differential impedance protection calculation can be enabled to be performed by newest electrical quantity, and the protection action speed is increased.