The present study assesses the linearity of a FTIR spectrometry using the blackbody. The least square method is used for the data fitting to gain the linearity of the spectral response of the spectrometry. The conception of “spectral linearity” is adopted and the curve diagrams of the spectral linearity is obtained for the facilities of DTGS and MCT detectors, which reflects the spectral linearity characteristics and the influencing factors of the facilities. The detector, spectral range, and energy range which meet the requirements can be determined by the method, and the internal and external factors influencing the linearity can be shown by the diagrams. It is very important in the application.