To realize fast and high precision tracking measurement of fiber optic gyroscope (FOG) eigen frequency, a eigen frequency tracking method based on sawtooth wave modulation is proposed. Based on the eigen frequency measurement theory of fiber-optic gyroscope from even times eigen frequency sawtooth wave modulation, the sawtooth wave modulation signal with nearly even times eigen frequency is applied on the phase modulator, and phase of the wave is modulated. The demodulated error signal intensity represents the degree of sawtooth wave modulation frequency away from even times eigen frequency. The frequency of sawtooth wave modulation signal is modulated by the intensity of error signal, and the error signal is set to zero. At this time, the frequency of sawtooth wave signal is equal to even times eigen frequency, and the bias modulation of square wave is located at the eigen frequency exactly. The experiment results show that the proposed method can realize tracking of fiber-optic gyroscope eigen frequency. Compared with the traditional measurement method, the proposed method have the advantages of speediness and high precision. The accuracy of the measurement is better than 1 Hz.