The invention provides a current blanking device of transformer iron core grounding current online monitoring equipment. The current blanking device comprises an iron core grounding current signal sampling conditioning module, a signal harmonic separation module, a separation signal phase shift module, a separation signal amplitude modulation and synthesis module, a current amplification output module, a signal frequency tracking module and a feedback control module. The invention further provides a current blanking method of the transformer iron core grounding current online monitoring equipment. According to the invention, the original current of an iron core grounding current monitoring device in operation can be offset and blanked; the calibrated iron core grounding current monitoring device can easily return to a zero current state; and the accuracy of a calibration process is improved.