Abstract: An active electronic tag and a signal modulation method are provided herein. The active electronic tag includes an antenna, a first amplifier, an automatic gain control circuit, a phase synchronization locking circuit, a control unit, a tag unit and a second amplifier. The antenna receives a detecting signal transmitted from a reader, in which the detecting signal is carried on a first carrier. The phase synchronization locking circuit generates a second carrier. The control unit generates a response signal, in which the response signal is carried on the second carrier. By utilizing a phase difference of the first carrier and the second carrier, the response signal cancels the first carrier on the reader so as to transmit the information stored in the active electronic tag.