In the measurement of the parameters of an infrared thermal imager,the radiation tem- perature difference received virtually by the thermal imager is not the original temperature difference because the radiation temperature difference can be affected by the optical reflectivity of each unit of the measurement system and the atmospheric transmission. Through the analysis of radiation energe transmission, the concept of"apparatus constant"of a two-blackbody parameter measurement unit is proposed and is simulated through modelling.The model can reduce the error caused by the radia- tion temperature difference trasmission in measurement and improve the measurement accuracy of the two-blackbody parameter measurement unit.