Fiber-optic current transformer (FOCT) is an important primary equipment in UHVDC transmission system. FOCT based on PZT modulation is a widely used modulation mode, in which the polarization-maintaining fiber delay ring is the most sensitive to vibration, that is one of the main errors of FOCT under vibration. In this paper, the influence of vibration on FOCT error is analyzed on mechanical and optical characteristics. It is pointed out that the external vibration will produce an additional modulation superimposed on PZT modulation, which will change the harmonic ratio of the output signal and introducing an error. Both theoretical analysis and experimental results show that the mechanical resonance characteristics of the fiber delay ring have a significant influence on the error. The resonance curve and calculation formula for multiple resonant points are obtained, and the error for different frequencies, different accelerations and different measured currents are given. The theoretical analysis is basically consistent with the experiment.