The invention relates to a grounding transformer impedance determining method. A grounding transformer is constituted by three single-phase transformers, and the number of turns of the primary side winding and the number of turns of the secondary side winding of the single-phase transformer are the same. The method is characterized in that impedance calculation can be realized by the positive and negative sequence equivalent circuit and the zero sequence equivalent circuit of the grounding transformer; the grounding transformer leakage impedance, the grounding transformer exciting impedance, and the positive sequence, the negative sequence, and the zero sequence total impedance of the grounding transformer can be determined. The grounding transformer impedance determining method is advantageous in that the method can be provided for the grounding transformer of the power distribution system, the impedance calculation of various sequences of the grounding transformer of the flexible DC power transmission system based on the MMC technology, and the reference can be provided for the action threshold of the zero sequence component of the grounding transformer.