Relative spectral response is an important index for evaluating the performance parameters of infrared detectors, which directly affects the user′s usage. Using the detectors in the middle and long wave bands as experimental devices, two different test methods using Fourier spectrometer and monochromator are analyzed and discussed. For the long-wave infrared detector, the rear cut-off wavelengths of the monochromator and Fourier spectrometer are 928 m and 925 m, respectively, and the front cut-off wavelength is 760 m. For mid-wave infrared detectors, the rear cutoff wavelengths of the monochromator and Fourier spectrometer are 486 m and 485 m, respectively, and the front cutoff wavelengths are 360 m. The results show that the front-cut wavelengths of the two test methods are completely consistent, and the rear cutoff wavelengths are within the allowable error range, thus verifying the consistency and accuracy of the relative spectral response of the two instruments. The principles of the two test methods are easy to understand and operate, the results are accurate, which provides a certain reference for the optimization of infrared detector performance.