The local thermodynamic equilibrum (LTE) and non-LTE limb radiances in the upper atmosphere are calculated for the main infrared (IR) spectral regions by line-by-line radiative transfer model, which is reference forward model (RFM). According to the relative deviation, the non-LTE effect on IR limb radiance is analyzed under day and night conditions and the altitude range of application of the LTE is identified. The comparison of the limb radiance between day and night is presented. The analysis results show that the non-LTE and LTE IR limb radiances in the upper atmosphere have a significant deviation and solar pumping in the day can increase the deviation. There are significant differences in the limb radiances for some spectral bands between day and night. In addition to the CO2 4.3 μm spectral band in the day, there isn’t significant deviation between the non-LTE and LTE below 60 km for other spectral bands and it can reach about 80 km for CO2 15 μm spectral band. The results lay the foundation for further study of non-LTE model in the upper atmosphere.