Abstract:The Band-averaged Extraterrestrial Solar Irradiance (BAESI) is the significant factor to convert apparent reflectance to radiance. In order to calculate precisely the BAESI value of FY-4A/AGRI, taking four similar sensors including COMS/GOCI from North Korea, GEOS-16/ABI form USA, and Himawari-8/9/AHI from Japan for an example for the experiment. The solar spectrum widely used including ASTM-E490, ASTM-G173-03, Kurucz, WRC, Wehrli85 and Thuilliersolar spectrum and every different sensor's spectral response functions were chosen to participate in the calculation of BAESI and then compare the estimated value with the standard values respectively. Results show that the Mean Absolute Error(MAE) and Root Mean Square Error(RMSE) of Kuruczsolar spectrum are both the smallest among all the spectrums, which is an ideal choice in the optional spectrums. Besides, compared with the standard values of the different sensors, Kurucz's estimated value's maxed errors are -2.39% (GOCI-band3),1.49% (ABI-band6), 4.28% (AHI-08-band6) and 4.42% (AHI-09-band6) respectively, which are all less than 4.5%. Hence, Kurucz is chosen to calculate the BAESI values of every band of FY-4/AGRI, and the uncertainty of the results is analyzed. The results have solved the problem between the apparent reflectance and the radiance. Besides, the method is beneficial to calculate BAESI of other geostationary satellites.