An IoT wireless power transfer sensor is provided, which includes a sensor circuit, a group of retrodirective searching antennas, a group of energy harvesting antennas and a communication antenna. The sensor circuit includes a switching unit, an energy converting unit, a power management unit and an energy converting and storage unit. The searching antennas are connected to the switching unit. When the switching unit is on, the searching antennas don't harvest the energy of an external device's scanning signal; they reflect the incident energy to the external device. The energy harvesting antenna is connected to the energy converting unit and receives the incident energy to charge the energy converting and storage unit. When the power storage level of the energy converting and storage unit reaches a first threshold value, the switching unit is consecutively switched between on and off to transmit a device information in back-scattered way to the external device.
