This paper investigates the influence of carrier signal frequency error on the demodulation phase of the fiber optic hydrophone with the demodulation algorithm of the phase generated carrier (PGC) and heterodyne. The proposed theory is demonstrated experimentally, with results showing that the frequency error between the carrier wave and local oscillator signal will influence the demodulated phase signal of the fiber optic hydrophone, which is consistent with the theory proposed. When the PGC algorithm is adopted, the error leads to a time-varying amplitude of the demodulated signal while a linear trend is superimposed on the signal demodulated by the heterodyne method.