In order to precisely measure infrared radiation characteristics of space target, a radiometric calibration method is presented based on the internal calibration blackbody and external natural star with known infrared spectral characteristics, and on the basis of the proposed method corresponding radiation measuring mathematical model is established. Internal calibration blackbody is used to measure the responsivity of each detector pixel involving the back half light path of the telescope, and the external natural star is used to estimate equivalent transmittance of the atmosphere near target region with the first half light path of the telescope. A large number of infrared radiation intensity measurement experiment show that the accuracy can be better than 15% within 3~5 microns band on the 1.2-m ground based telescope when the target has strong infrared energy reference star and the exoatmosphere irradiance is higher than 1×1016 W/cm2 order of magnitude. The method can overcome the effects of the atmospheric path radiation, extinction and large diameter radiation calibration source on the measurement precision about infrared radiation intensity of exoatmosphere space target, and it is suitable for calibrating of the ground based infrared radiation measurement system with large diameter and measuring of the infrared radiation characteristics of exoatmosphere space target with high precision.