A case of mal-operation of a transformer differential protection caused by another paralleled no-load transformer switching on is analyzed. The inrush currents caused by switching on of a no-load transformer result in the variation of current transformer's transfer characteristics， which leads to a decline of the second harmonic in the differential current， so the second harmonic blocking scheme is invalid， resulting in mal-operation of the differential protection. Some countermeasures are proposed to prevent the differential protection from mal-operation by taking into account the field conditions.