The temperature-field measurements obtained by endoscopic infrared thermography for gas-insulated electrical equipment are the combined result of target radiation, background radiation, and gas-selective absorption within the equipment enclosure. Current infrared temperature-calibration methods are mainly developed for atmospheric environments and do not account for the multiple reflections of thermal radiation between cavity surfaces or the energy attenuation caused by short optical paths and high-pressure gas atmospheres. To address this issue, this paper proposes an apparent-temperature compensation method for infrared thermography in closed cavities under specific selectively absorbing gas atmospheres. First, based on a single-path thermal-radiation reflection-transmission model, spectral transmittance calculations and finite-element simulations of gas-state distributions in closed cavities are conducted. The influences of radiation wavelength, gas pressure, gas temperature, and optical path length on thermal-radiation transmittance are analyzed, leading to the establishment of an analytical model for the thermograph′s apparent temperature under composite paths. Subsequently, heating experiments of electrical equipment under air and SF6 atmospheres are carried out. Model parameters for different temperature-field distributions and gas pressures are derived from the measured data, enabling temperature inversion under different gas conditions. The results indicate that the gas transmittance can be approximated by its initial value after equipment inflation even during temperature rise, The gas transmittance between discrete target and background surfaces in the cavity can be treated as a constant, and only the energy attenuation along the optical path before the first radiation reflection needs to be considered. Finally, the mean absolute error of apparent-temperature prediction under different gas atmospheres is less than 1 K. The findings provide a concise computational model for rapidly quantifying the effects of air and SF6 atmospheres on endoscopic infrared temperature measurement in closed cavities.