This paper presents a novel method to improve the subcarrier frequency accuracy of tag chip for UHF RFID sensing system based on EPC global Gen2 protocol. By changing the tag's response to the Write command, the tag returns the subcarrier before returning the response specified in the protocol. In this way, a frequency calibration loop will be established between reader and tag. This method not only enables a low-power oscillator with a precise frequency to be integrated into a tag chip, but also enables flexible configuration of the frequency. The tag chip containing this idea is fabricated in SMIC 0.18 μm CMOS process with a die size of 850 μm × 850 μm. Measurement results show that the proposed novel method can realize the calibration of the subcarrier frequency, and the subcarrier frequency deviation is less than ±0.385KHz in the range of 40KHz-640KHz. With this accuracy, the tag can be allocated up to 78 subcarrier communication channels.