Infrared image emulation is useful in the development of infrared systems. To get a full theoretical model of infrared image emulation, the mechanism of those four parts such as object and background radiation's emulation, atmosphere's emulation, optical system's emulation and staring detector's emulation is further analysed. Finally, an emulation picture is generated to show the effectiveness of the emulation model. To get good effectiveness, the ununiformity and diffraction effect are especially considered in the emulation picture.