Bidirectional reflection distribution function(BRDF) measurement uncertainty of the solar diffuser has direct impact on the on board absolute radiometric calibration accuracy. Conventional BRDF measurement apparatus is difficult to meet the requirements of on board absolute radiometric calibration in terms of measurement accuracy and function. In this context, a set of BRDF absolute measuring system was established. In the system, the high brightness, high uniformity integrating sphere radiation are used as light source, high-precision six-axis robot and indexing plate are used as BRDF gonioreflectometer, and wide spectrum of large dynamic range of the photoelectric detector is used as signal detection unit. Through geometry, electronics and other related physical quantities of high precision calibration and traceability, BRDF measurement uncertainty of solar diffuser less than 1% is achieved.