In order to improve the on-orbit calibration accuracy of a CO2 spectrometer, the principle of on-orbit radiation correction was established, and key aspects such as the fabrication, BRDF calibration, and application of a diffuser were systematically studied. Based on the working principle and system composition of the CO2 spectrometer, the device and strategy for performing the on-orbit radiation calibration were introduced, and the fabrication method and optimization of the process parameters of the diffuser were established. Next, an accurate calibration method with lamps and detectors as transmission standards for the diffuser's BRDF are developed. The reference BRDF, angle correction factor, and hemispherical reflectivity of the diffuser were tested. In addition, the calibration accuracy in the laboratory was analyzed and subsequently verified by the application results obtained during the initial stage of the orbit. The results obtained before the launch show that the calibration accuracy of the diffuser was better than 3% for all three bands at 760 nm, 1 610 nm, and 2 060 nm. The initial test results on orbit indicate that the accuracy of the absolute radiation calibration of the CO2 spectrometer at 1 610 nm was better than 5%. These results can satisfy the accuracy requirements for the diffuser radiation calibration of the CO2 spectrometer.