Abstract: To overcome the difficulties in recognizing the working pattern of the reflectance spectrum of the fiber Bragg grating (FBG) and in meeting the requirements of the temperature measurement accuracy in the space vacuum and the thermal cycling environment, a method for diagnosing the abnormal pattern of the FBG reflectance spectrum is proposed, with the acquired wavelength-temperature curve adaptively fitted in a piecewise way. Experimental results show that the method is effective in the accurate recognition of the abnormal pattern of the FBG reflectance spectrum both at the normal atmospheric temperature and in the severe vacuum thermal environment. The fitted results of the wavelength-temperature curve show that the fitting errors are as small as 0.052 126 and 0.017 083, which meet the accuracy requirements of the FBG sensor under the thermal vacuum condition.