A nuclear power plant earth fault detecting method and device includes acquiring first current and second current corresponding to a positive lead and a negative lead at the same time in the positive lead and the negative lead at multiple predetermined positions in a branch of a power distribution system; monitoring and judging whether the first current is identical to the second current or not in real time; when the first current and the second current are not identical, acquiring the location of the positive lead and the negative lead where the first current and the second current are not identical and sending the failure information of the location. When the first current and the second current are not identical, near-load end failure can be determined at the location corresponding to the first current and the second current in the leads. Failure locations can be determined quickly according to the predetermined locations and failure location checking accuracy can be improved distinctively, and quick and effective failure elimination is facilitated.