A short circuit protection device for the protection of an alternator in which turning on/off a trigger switch causes conduction/cutoff of the field current of the field coil of the alternator is disclosed. The protection device enables the field current to pass through a sensor so that when the field current is abnormally high and the voltage drop across the sensor rises, the potential of an input end of a boost circuit electrically connected to the sensor correspondingly arises and an output end of the boost circuit electrically connected to a shutoff device drives the shutoff device to turn off the trigger switch, and the boost circuit further keeps the shutoff device in on-state to hold the trigger switch off till the short circuit condition is eliminated and the protection circuit system is reset.