The utility model relates to a demodulation device for the wavelength of fiber grating, which adopts the structure that a scanning light source is connected with a circulator, the circulator is connected with the fiber grating, a photoelectric conversion and signal amplification processing module is connected to the circulator to receive light signals reflected by the fiber grating; the photoelectric conversion and signal amplification processing module is connected with a differentiation zero-crossing detection module and a comparison module respectively, the differentiation zero-crossing detection module and the comparison module are connected with a data processing module respectively, the data processing module is connected to an adjustable benchmark module, and the adjustable benchmark module is connected to the comparison module for providing the comparison module with benchmark value for the comparison. The demodulation device solves the fiber grating side peak process problem and large data operation amount problem.