Abstract: A method for authenticating a passive RFID tag includes acquiring a tag fingerprint of a first tag as a first tag fingerprint, the first tag being the genuine tag; acquiring a tag fingerprint of a second tag as a second tag fingerprint, the second tag being the tag to be authenticated; comparing the first tag fingerprint with the second tag fingerprint: if the first tag fingerprint is consistent with the second tag fingerprint, determining that the second tag is a genuine tag, otherwise determining the second tag is a forged tag. The tag fingerprint is the persistence time enabling the passive RFID tag to operate normally during discharge after fully charging. The beneficial effects include being high in robustness to the change of environment and high in authentication accuracy and capable of being directly deployed on an existing commercial RFID device without modifying hardware of the tag and reader.