Abstract: Spectral irradiance, which is the most elementary parameter in spectral radiation metrology, is the important foundation on studying the characteristics of all kinds of radiating sources and optoelectronic detectors. In order to improve measurement precision and traceability of spectral irradiance quantity value, the optical metrology station uses BB3200K high temperature black body as a fiducial source and adopts the new method for measuring the radiation temperature. We derived the relation between radiation temperature and radiation power by making use of a series of radiation theories and geometry principle. The precision quantity value measurement of spectral irradiance is realized by absolute measurement of the radiation power by means of a cryogenic radiometer. The measurement results are shown in the paper. The theory laid the foundation for the new detectorbased quantitytransfer system. This paper mainly introduces the new measurement method and math model about measuring spectral irradiance. At the same time, the measurement device and measurement principle are introduced.