The invention provides a magnetically balanced series transformer zero sequence differential protection method and a magnetically balanced series transformer zero sequence differential protection device, which consider the current characteristics of a balance winding when a slight turn-to-turn fault occurs in the series transformer area, and construct differential protection based on a magnetic balance principle. Current transformers are installed on a grid side winding, a valve side winding and the balance winding of the series transformer, self-generated zero-sequence currents of the currenttransformers are calculated, differential currents are selected from the self-generated zero-sequence currents and serve as calculation quantities, and the slight turn-to-turn fault of the transformer windings can be reflected sensitively. The magnetically balanced series transformer zero sequence differential protection device can reflect the slight turn-to-turn fault in the series transformer area sensitively by taking comparison situations of the self-generated zero-sequence currents of the balance winding, a rated current of the balance winding and the zero-sequence currents of the otherwindings, effectively prevents protection malfunction when an external fault occurs, and can be widely applied to the series transformer containing the balance winding.