A double square wave and B-spline wavelet for demodulation of a Weak Fiber Bragg Grating (WFBG) was proposed and demonstrated. A period of single square wave is set as the round trip time of laser transmission in the fiber between two adjacent WFBGs. The burst operation is conducted to the single square wave to form a double square wave, for which the rear square wave reflected by former WFBG and the front square wave reflected by latter WFBG overlap and interfere. B-spline wavelet transform is used to reduce the noise of interference signal. Hilbert transform is applied to produce π/2 phase-shift of the interference signal. Arc tangent operation is conducted to the ratio of the interference signal with the phase-shifted signal to obtain the phase signal of the interference signal. A 5-WFBG array with 50 meters equispaced length is put on a wooden platform, and received sinusoidal sounds with different amplitudes and frequencies respectively. The experimental results can reflect the information of the sounds well. The advantage of the proposed demodulation method is that the optical structure and data processing is simple.