Abstract: A study on the radiometric calibration of large entrance pupil space telescope is made. After analyzing the difficulties on this work, a new calibration method based on the improved on-board blackbody source calibration in FY-2C is put forward. This new method overcomes the limitation in field of view and diameter of the entrance pupil; and it realizes the full light path calibration to telescope with any possible entrance pupil. Finally, based on the uncertainty analysis of the new method, the advantages of the new method are given.