Based on the actual optical system parameters of the calibration spectrometer (SCS), the source of the radiance non-uniformity of the solar diffuser (SD) attenuated by the solar attenuation screen (SAC) is analyzed. Based on the physical model of the SD spectral radiance in orbit while calibration, combined with part of the parameters measured in the laboratory, the variation law of the SD’s radiance versus the angle of incidence (AOI) during the calibration measuring in a year is obtained. And comparing with the variation law of the SD radiance versus AOI set on SCS mearsured with a small divergence solar simulator as the source for illuminating in the laboratory, the correctness of the physical model of spectral radiance at the calibration time of the SD in orbit is verified. The emission radiance obtained by the illuminating SD can achieve energy non-uniformity in the SCS focal plane of better than 0.47%/(°), which satisfies the requirement that the SCS relative radiation calibration has a focal plane uniformity better than 99.5% at the working area of the radiation source. Finally, according to the actual application state, the uncertainty of the on-board spectral radiance standard surface source value formed by the SAC+SD method can be better than 2.13%.