In order to realize the wavelength demodulation of the optical fiber sensor, a 8 channel 50 GHz dense wavelength division multiplexer was used as the demodulation tool of the fiber Bragg grating pressure sensor. Taking the center wavelength of each channel as the calibration standard, the optical power output obtained from each channel was utilized to acquire the envelope of the distribution curve of the fiber grating reflection spectrum. By using the Gauss polynomial fitting method as peak-seek algorithm of fiber grating wavelength, the accurate position of the center wavelength of the fiber grating reflection spectrum was obtained and a simple structure, low cost, all-fiber wavelength demodulation system was achieved. The experimental results show that the linear fitting degree of the system pressure and wavelength is 0.996 5, the measured wavelength resolution is 0.3 pm, the maximum reference error is 2.6%, and the measurement accuracy is 0.16 MPa.