The invention discloses a method of identifying excitation inrush current on basis of normalized area indexes of a variable data window. According to the method, threshold +Epsilon and threshold -Epsilon are set above and below a time axis, respectively, by means of the variable data window judging algorithm, and a start point of and an end point of the variable data window are determined by means of the thresholds and an extreme value of certain sampling current; in the variable data window selected, according to a sampling interval and data window length, a standard sine curve is virtualized frontward and backward from the extreme value, normalized area indexes of differential areas of the standard sine curve and the sampling current are compared, and thus protection locking and unlocking is achieved. The method has the advantages that inrush current waveform and fault current waveform can be distinguished to the maximum extent, differential protection can be locked quickly and reliably in case of excitation inrush current of a transformer, the differential protection is never mistakenly locked in case of an internal fault, the differential protection can be quickly unlocked in case of no-load closing of the transformer with fault, and transformer saturation resistance is higher.