The invention discloses a method for checking a differential protection secondary circuit of a transformer, and belongs to the field of differential protection of an electric power system. Through a mode of arranging a three-phase large current generator on the low voltage side of the transformer, the checking method solves the problem that an on-load test of secondary connection of a differential protection current transformer of the transformer cannot be carried out because the load current of the conventional to-be-protected transformer is too low. The checking method judges whether the parameter setting of a differential protection device is correct by means of comparing the induced current, acquired by the differential protection device, of a secondary winding of the current transformer with the theoretical value. The checking method judges whether the connection of a differential protection secondary circuit is right according to the condition that whether the differential current is 0. The checking method judges whether the polarity of the current transformer of the differential protection secondary circuit is correct according to the condition that whether the induced current of the secondary winding of the same-side three-phase current transformer is in a positive phase sequence. The method is suitable for checking the differential protection secondary circuit of the transformer.