The signal demodulation of fiber optic hydrophone based on the 3×3 coupler method is generally affected by the asymmetric characteristic of coupler, which leads to inaccurate demodulation results and the measuring signals. To solve the problem of demodulation errors caused by both the amplitude and phase difference, which are commonly existed for a 3×3 coupler, a method of amplitude and phase demodulation correction based on differential cross-multiplying is proposed. Through the least squares ellipse fitting pairwise of three asymmetric interference signals, the amplitude and phase parameters of the interference signals are obtained, and the amplitude asymmetry is eliminated by DC removal and normalization. Further, the differential cross-multiplying operation is performed pairwise for the obtained three signals, which contain asymmetric phase. Then, the phase and the related correction coefficient of the signals are calculated by trigonometric transformation. In this way, the accurate demodulation results of the fiber hydrophone are achieved. Comparison of the proposed method with the existed method which only considers the asymmetry of amplitude is conducted. The demodulation errors of two methods varying with frequency and amplitude of the input acoustic signal are analyzed by numerical simulations. Results show that the proposed method could obtain the demodulated signal which is closer to the nominal signal waveform and has smaller demodulation errors. Moreover, demodulation experiments of fiber optic hydrophone are also implemented in the frequency range of 5 ~ 30 kHz in an anechoic water tank, which illustrate the effectiveness and stability of the presented correction method for demodulation.