Thermal emission from Infrared Radiation (IR) instrument is an important background source
which will reduce Signal-to-Noise Ratio (SNR) of the system. It is necessary to estimate background radiation due to instrument's thermal emission in opt-mechanical design phase. An analytic model about thermal emission of reflect/refract combined optical system is established here
which is used to calculate background of space IR systems. Object signal is compared with background to show how background affects SNR. Finally