The present paper introduced a spectral calibration method to calibrate a large aperture infrared radiation facility. We developed a radiometer which consisted of optical system, infrared detectors(cavity pyroelectric detector and HgCdTe detector 2～14 μm), fine mechanical modulator, lock in amplifier, signal processor, etc. At first, we analyzed the method on how to measure the spectral calibration of the large aperture infrared radiometer, and established the spectral calibration facility. Then, we tested the nonlinear response for the cavity pyroelectric detector and HgCdTe detector. Finally, we used the cavity pyroelectric detector to calibrate the relative spectral responsivity of HgCdTe detector at several wavelengths on the facility. Through the comparison of the two methods for measuring the relative spectral responsivity, the average of multiple measurements and comparative analysis of two methods were given. The uncertainty analysis of the whole system showed that the measurement uncertainty of the facility was better than 3.4%.