A new measuring method for fiber ring eigenfrequency was presented. Different from the common square wave modulating, the new method is based on the sine wave modulation, so the problem of dead zone was solved. The output signal of fiber optic gyroscope was analyzed when using sine wave modulation, quadratic relationship was obtained between second harmonic and modulation frequency, eigenfrequency can be obtained by calculating the parabolic vertex. On the basis of the new method, a test system was developed by updating fiber optic gyroscope system. Test result shows that the measuring precision of 10 ppm can be achieved, which shows that the proposed method is a method with low cost and high precision method for fiber ring eigenfrequency.