The principle of on-board calibration for a solar diffuser was expounded, and a method to monitor the degeneration of the solar diffuser was researched. A Solar Diffuser Degeneration Monitor(SDDM) was designed to monitor the degeneration of Bidirectional Reflectance Distribution Function(BRDF) of the solar diffuser by measuring the ratio of the reflected radiance of solar diffuser to sun irradiance. The relationship between on-board calibration and the monitor of the solar diffuser was described. An integrating sphere with two light imports and a baffle for the SDDM was designed to eliminate the dependence of the changed incident angles on the detector response. According to the monitor process, the physics models of SDDM were given, and the dynamic range and Signal to Noise Ratio(SNR) of detectors in the SDDM were validated and analyzed. Finally, the illumination of the sun on the SDDM was simulated in a lab to test the transmittance model repeatedly for several times. By analysis of the measurement uncertainty related to the Bidirectional Reflectance Factor(BRF) of the solar diffuser, satellite attitude and the mounting error of the SDDM, it shows that the monitoring uncertainty of the solar diffuser is better than 0.88%.