Solar diffuser reflectance degradation monitor (SDRDM) is used to gain the reflectance of a solar diffuser (SD). The transmittance of the sun view port and the relative bidirectional reflectance distribution function (BRDF) of the solar diffuser vary with the angle. Therefore, the SDRDM observation signals are affected by the angle of the incident light. The effect factor is called the SDRDM angle factor (abbreviated as the angle factor). This study proposes a method for calculating the SDRDM angle factor using on-board data employed on the premise that the solar diffuser plate does not decay in the early orbit stage. The same solar angle ratio calculation is used, thereby verifying the stability of the solar diffuser reflector without discussing the angle factor. The angle factor is calculated and compared with laboratory measurements. Results show that the deviation between the calculated values and the measured values of the laboratory is less than 0.5%. The angle factor measured prior to the launch can be used to calculate the BRDF degradation factor of the SD.