A multi-channel scanning imaging radiometer is installed on the FY-4 meteorological satellite. To realize high-precision quantitative remote sensing, an on-board calibration device is provided. The device primarily uses a solar diffuser (SD) to reflect sunlight as a standard radiance source, and the solar diffuser reflectance degradation monitoring (SDRDM) device regularly monitors the reflectance variation of the SD. This study introduces the working principle of SDRDM and establishes a calculation model of the bidirectional reflectance distribution function (BRDF) degradation factor of the SD based on the characteristics of on-orbit data. The data show that the SDRDM device is affected by temperature and its uncertainty is 0.165% per ℃. The ratio monitoring method eliminates the influence of temperature. The combined monitoring uncertainties of three channels' degradation factors are calculated, which is 1.48% (k=2) for the first channel and 1.16% (k=2) for the second and third channels, respectively. These results suggest that the SDRDM device can effectively monitor the degradation of the SD's reflectance.