Abstract: Phase modulation spectroscopy is an important method of signal detection in resonator optic gyro. By the use of expansion of Bessel function and optical field overlapping method, the transmission properties of optical fiber ring resonator was analyzed. A testing system of resonant properties was set up. Based on the LiNbO3 phase modulation, the experiments to research resonant characteristics and demodulation curve features was carried out under different frequencies and voltages of modulation, and the experimental result was analyzed, then the relation of frequency and voltage of modulation and resonant and demodulation signal of fiber ring resonator was obtained. The splitting phenomenon of resonance dips during the experiment was measured and analyzed. By fitting the data, the relation of critical frequency and voltage of modulation of splitting phenomenon of resonance dips occurs was obtained.