The invention discloses a fault judgment method and a backup protection method for a medium-voltage side dead zone in a main transformer. A fault point dead zone range is determined by finding fault characteristic values, the fault characteristic values are collected and analyzed by a computer, a tripping instruction is sent, three side circuit breakers of the main transformer are switched off, and fault points are eliminated; a bus differential protection jump signal is output through an internal signal module in the bus differential protection device, is connected with a transformer protection device through a line and enters a medium-voltage side backup protection logic module, the main transformer protects the inside and provides a medium-voltage side circuit breaker TWJ circuit and a TA current circuit, the circuits are connected with a portal and a time relay in series and are connected with a main transformer operation box through secondary lines to trip the circuits, and tripping is achieved through pressing plates at three side outlets. According to the methods, the fault range is determined according to displacement situations of a main transformer voltage side circuit breaker and a TA secondary current value after 110kV bus differential protection when faults occur, corresponding action delay is set, three sides of the main transformer are tripped, and impact time of short-circuit currents to a main transformer body is greatly shortened.