Portable charger capable of automatically judging polarity of power supply

The invention discloses a portable charger capable of automatically judging the polarity of a power supply. The portable charger comprises a portable charger (1), a portable charger output switch (2) and a protective relay action contact (3), and is characterized in that the protective relay action contact (3) is connected with a polarity detection judgment device (4); the polarity detection judgment device (4) comprises a starting module (4-1) of a protective relay, a polarity detection judgment module (4-2) and a lamplight and sound alarm module (4-3); the starting module (4-1) of the protective relay, the polarity detection judgment module (4-2) and the lamplight and sound alarm module (4-3) are connected with a direct-current system electric power bus (5) of a transformer substation and the protective relay action contact (3) respectively. The portable charger has the advantages of capabilities of alarming in time and ensuring personal and equipment safety, easiness in wiring, small workload, safety and r