Abstract: For the longterm monitoring and correcting of the radiometric performance of the imager, the expandable diffuser used for calibration in full FOV and full optical path method was designed, considering the characteristics and requirements of its wide spectrum range, high accuracy requirements, large aperture and long service life.The solar diffuser is installed on the front side of the optical system and does not affect the normal imaging of VIMI.When VIMI needs calibration，the diffuser is expanded to the front of the optical system via the driving mechanism. According to the characteristics of the GF5 satellite orbit, the requirement of the calibration energy and the installation matrix of the imager relative to the satellite, the expansion angle of the diffuser is 39°. The 430mm×430mm largesize PTEE diffuser is manufactured to ensure full FOV and full optical path calibration. The diffuser’s directional hemispherical reflectance is higher than 95% when the spectrum ranges from 420nm to 2400nm， and variation of  BRDF in the direction of imager observation is better than 25%.The diffuser stability monitoring radiometer is designed to monitor the onorbit attenuation performance of the diffuser, and the monitoring accuracy is 15%. The accuracy of onorbit calibration is 476%, which meets the requirement of index.

Key words: onorbit calibration, solar diffuser, sun zenith angle, radiometric calibration, GF5 satellite