A method for compensating the axial ratio of an antenna for testing radio-frequency identification (RFID) tags and an apparatus using the method are provided. The method includes the following steps. An initial location of the RFID tag (or a tagged product) is set to obtain an initial vector. The RFID tag and the antenna are respectively rotated with a first axis and a second axis to detect characteristics of the RFID tag in all directions, and the first axis and the second axis are perpendicular. A polarization angle of the RFID tag is calculated according to the initial vector of the RFID tag and a location of the RFID tag after rotation. A compensation value is obtained through a look up table according to the polarization angle, so as to compensate for the axial ratio of the antenna.