Abstract: The theoretical formula between the reflection phase retardation difference of S polarized beam and P polarized in square ring resonator and the beat frequency driving voltage difference in the piezoelectric ceramic (PZT) was derived. The frequency response characteristic of S polarized beam and P polarized beam in the square ring resonator was analyzed, and a new method for measuring the phase retardation difference by measuring the voltage difference was proposed. The passive ring resonator frequency response measuring system was set up, and the validity of the theoretical analysis was verified by experiment. The results indicate that the measuring error is less than 1.5%, which satisfies the requirement for the reflection phase retardation measurement.