The invention relates to a method for judging and protecting low voltage in transformer short circuit fault, which comprises the following steps of: (1) measuring a phase voltage on one side of a wiring transformer by a protection device; (2) calculating a magnitude of line voltage between each two voltages on one side in the step (1) by the protection device; and (3) respectively comparing the magnitude of the phase voltage and the magnitude of the line voltage on one side in the step (2) with a setting value to form low voltage criterions, and forming an or gate output by all the low voltage criterions. After the application of the method, when all sides of the transformer are in a state of interphase short circuit and a neutral grounding system is in a state of single-phase grounding, one of the measured voltages is bound to be zero, the setting value can take a low value so as to ensure that a low voltage starting criterion can still have high action sensitivity and improve the reliability of non-maloperation during normal operation; and the low voltage setting value can enable a current setting value of an overcurrent criterion to take a low value so as to improve the action sensitivity of the overcurrent criterion. The invention has few channels to be protected and simple hardware.