The theory of on-orbit reflectance calibration of the solar diffuser panel is expounded and the physical model of on-orbit reflectance calibration is established. As the precision of on-orbit calibration is affected mostly by the degeneration of the solar diffuser’s bidirectional reflectance distribution function(BRDF), the method of monitoring the degeneration of BRDF is discussed. According to the monitoring physical model of the solar diffuser stability monitor of the moderate resolution imaging spectroradiometer (MODIS) and the problem founded in the on-orbit performance evaluation, an optical structure of integrating sphere with two imports for light and a baffle inside is proposed. The parameters in its physical model are decreased and some factors that bring uncertainties are cut down while the function can be ensured. It is validated that the optical structure can prevent the sun view response from the ripple signal response in experiment. Based on the physical model of the new optical structure and the domestic testing level, the uncertainty of this system is estimated, and it can be less than 0.68%.