Abstract: In the application of passive RFID tag, a new method of calculating tag distance by utilizing phase difference was proposed. The relationship of the time and phase difference which was produced by the read-write signal of the passive tags in the process of transmission is of great use to calculate the time required by the signal in the transmission, so as to measure the distance. Frist the simulation model was established in MATLAB for theoretical validation of this phase difference method, then real experiments were used to the verification. The range error was calculated for the reader-tag distance below one wavelength and more than one wavelength in the theoretical simulation and experiments. The experimental results showed that the relative range errors were less than 6% for distance below one wavelength. For the reader-tag distance more than one wavelength, the shorter was the distance, the higher was the measuring accuracy, and the relative range errors were less than 10% in the range of 918~924 MHz.