Abstract: Low-slow-small (LSS) unmanned aerial vehicles (UAV) have a low flight altitude, slow flight speed, small size, and complex reflection environment radiation, which makes it difficult to calculate their infrared radiation characteristics. In this study, we introduce the bidirectional reflectance distribution function (BRDF) to construct an engineering calculation model of the LSS UAV's own radiation and reflected environmental radiation. A program for calculating the infrared radiation characteristics of an LSS UAV was developed. Using the measured infrared radiation intensity data of the DJI Phantom 4 Pro UAV from the University of Central Florida, the model was verified and analyzed using different observation angles and wavebands (MWIR and LWIR). The verification results show that the calculated data of the model are in good agreement with the measured data and can be used in practical engineering applications.