The invention relates to a grounding protection system and method of a box type transformer of a wind power plant. The grounding protection system comprises a current collection module, a current analysis module and a circuit breaker control module; the current collection module is lap jointed on the surface of a connecting cable on the low voltage side of the box type transformer and used for obtaining a phase splitting current on the low voltage side of the box type transformer and converting the phase splitting current into a secondary phase splitting current; the current analysis module is used for receiving the secondary phase splitting current, and outputting a tripping signal when the secondary phase splitting current is greater than a current setting value; and the circuit breaker control module is used for receiving the tripping signal and controlling a circuit breaker arranged between the box type transformer and a wind turbine generator unit to perform tripping. When the connecting cable on the low voltage side of the box type transformer suffers from grounding fault, the secondary phase splitting current transferred to the current analysis module by the current collection module is greater than the current setting value, the current analysis module outputs the tripping signal, and the circuit breaker control module receives the tripping signal and controls the circuit breaker between the box type transformer and the wind turbine generator unit to perform tripping.