Abstract: Wavelength demodulation is one of the key technologies in the field of fiber grating sensing system. A method based on DFB laser which wavelength can be modulated is introduced in this paper. By controlling linearly the temperature of DFB laser tube, the wavelength of DFB laser can be modulated. By experiment and analysis, the relation between the tunable wave-length of DFB laser and the tube temperature is presented. With the advantage of the relation, the temperature detection system with fiber grating is established with an error margin of less than 1℃, and the result of experiment indicates that the new method of demodulating wavelength is a useful applied in detecting temperature.