A new approach was presented to solve the problem of fiber optic gyro（FOG）eigenfrequency auto measuring.Based on the modulation-demodulation theory，the paper adopted symmetric square-wave to modulate Y-waveguide and analyze the demodulated signal.Then the relationship between the modulation frequency and duty cycle of demodulated signal is got.To the duty cycle as a feedback to adjust modulation frequency，when the duty cycle is 50% the modulation frequency is half of eigenfrequency.And this method is applied in our test system，which shows that accuracy is about 0.01kHz，and auto-test can finished in several seconds.Compared with existing methods this auto-test method is more accurate，rapid and easy to implement automatic digital measurement.