To reduce the data acquisition quantity and improve the accuracy of zero difference symmetric demodulation, a method for extracting interference signals in a time-division multiplexing system with a weak fiber Bragg grating (WFBG) based on Michelson interference is proposed. Continuous wavelet transform is applied to the intensity signal to find the extreme value of the transform coefficient. The adjacent minimum and maximum values are used to define the laser interference region. According to the laser pulses with different width, the wavelet scale factor is adjusted to find the interference region. The interference signal is obtained based on the maximum intensity and the average intensity in the interference region, respectively, and the phase signal of the interference signal is demodulated by the zero difference symmetry algorithm. The WFBG sensor is put into a vibrating liquid column. The same weak sinusoidal signal is measured repeatedly, and the measured signals are fitted by the sinusoidal curve. The fitting results show that extracting the short pulse interference region by the continuous wavelet transform and calculating the interference signal by the average value method are helpful to improve the zero difference symmetric demodulation accuracy of the sensing system.