The simulation algorithms for limb infrared radiation in the middle and upper atmosphere are investigated using strategic high-altitude radiance code (SHARC) and combined atmospheric radiative transfer (CART) model. The results confirm the availability of CART limb model preliminarily. The comparison shows that local thermodynamics equilibrium(LTE) model is effective under at least 45 km tangent altitude for the main limb infrared radiance simulation, while non-local thermodynamics equilibrium(non-LTE) model should be applied starting from different altitudes for the different infrared bands and atmospheric conditions. For 15 μm band, LTE model can be effective under at least 80 km tangent altitude; but for 4.3 μm band and 9.6 μm band, non-LTE model should be applied about above the stratosphere.