The present disclosure provides a circuit and a method for improving energy harvest for an RFID tag with a temperature sensor, where an instruction command sent by a card reader includes a modulated part to invoke temperature sensor functions, and an unmodulated constant-envelop RF signal part with an extended time of duration to charge a switched additional energy storage capacitor embedded in the circuit. The switched additional energy storage capacitor is connected to the circuit upon a mode control signal corresponding to the sensor operation mode of the RFID tag. Thus, the RFID tag with the temperature sensor is ensured to conform to the timing window protocol for regular downlink operations, and at the same time, is capable of meeting higher energy demand for a high accuracy temperature sensor operation.