A mobile device, an authentication device and authentication methods thereof are provided. The mobile device displays a QR code on its display panel, and continuously transmits a wireless signal carrying a hardware serial number of the mobile device. The QR code stores service data including an identification code assigned to the mobile device and the hardware serial number of the mobile device. The authentication device captures the QR code from the mobile device and determines whether there is a received wireless signal carrying the hardware serial number of the mobile device. If there is a received wireless signal carrying the hardware serial number of the mobile device, the authentication device determines that the QR code passes authentication.