The piezoelectric transducer(PZT) is often used to provide unreciprocal phase modulation in an open-loop fiber optic gyroscope(FOG), but the non-linearities can affect the stability of work point and induce the Saganc phase error.In this paper, the spurious modulation signal output caused by the second harmonic phase modulation and phase delay is analyzed in theory.Numerical results demonstrate that it would be better choose the second and fourth harmonic components for the small Sagnac phase, while the fir...