The on-board calibration unit is an important part of the space-borne remote sensor and used to achieve the on-board radiometric calibration and other functions. An on-board calibration unit installed on a time-modulated fourier transform spectrometer was introduced. Used the sun as the light source to illuminate the calibration diffuser, the on-board calibration unit carries out the on-board radiometric calibration of the spectrometer in the full aperture, the full optical path and the full field of view. The calibration diffuser was in the front of the optical path of the spectrometer to reflect the sunlight and the standard radiance was obtained by used the known exoatmospheric solar irradiance and the BRDF(bidirectional reflectance distribution function) of the calibration diffuser. The BRDF of the calibration diffuser needs to be measured accurately in the lab. With the whole spectrometer, using the sun simulator and the standard diffuser, the in-lab system-level BRDF test method of the calibration diffuser was introduced. While using the system-level BRDF of the calibration diffuser, the accuracy of the on-board absolute radiometric calibration was analyzed and can satisfy the requirement of ≤5%.