A theoretical calculation model of scarfskin, the nozzle of the turbojet engine and exhaust plume for nonafterburning unmanned aerial vehicle (UAV) is built. The infrared radiation calculation formulas are educed under different observation orientation and elevation by the finite element analytical method. Both the distribution of temperature and pressure and the CO2, H2O concentration field are numerical simulated by the software Fluent, which assure that the numerical value is precise. The atmospheric transmissivity under different observation elevation angles is analyzed by LOWTRAN. The experimental results for a given UAV indicate that for the scarfskin radiation, 3.7～4.8 μm spectrum band can be ignored, so the 8～10 μm spectrum band is the main spectrum band. The distributing rule of radiation for the nozzle of the turbojet engine and exhaust plume for the 3.7～4.8 μm spectrum band is the same as that of 8～10 μm spectrum band. At the same time, the numerical value of the 3.7～4.8 μm spectrum band is larger than that of 8～10 μm spectrum band. The max radiations of scarfskin and exhaust plume are showed at the orientation angle of 90°and elevation angle of 45°.