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Owing to limitations imposed by the experimental requirements, it is difficult to carry out pressure-volume-temperature experiments on CO 2 -containing natural gas in high-temperature and ultrahigh-pressure gas reservoirs. Relevant research is also insufficient, which has led to a lack of clarity in current understanding of the microscopic mechanism of variations in the deviation factor of high-CO 2 natural gas under high-temperature and ultrahigh-pressure conditions. This has greatly limited the development of natural gas reservoirs containing CO 2 . To reveal the microscopic mechanism of variations in the deviation factor of natural gas containing CO 2 as a function of pressure under high-temperature and high-pressure conditions, by physical simulation experiments, the deviation factors of samples of sour natural gas with known CO 2 contents from the Ledong gas reservoir were determined. Then, according to the idealized parameters of the physical experiment, a molecular model of natural gas containing CO 2 was established using molecular simulation methods. Changes in molecular density, molecular volume, nonbonding interaction energy, potential energy, and kinetic energy during variations in the deviation factor of a CO 2 -containing natural gas system as a function of pressure under high-temperature and ultrahigh-pressure conditions were quantitatively studied. Using molecular simulation techniques, it was found that the changes in total energy, kinetic energy, and potential energy between molecules are the internal factors that cause variations in the deviation factor of natural gas systems containing CO 2 under ultrahigh-temperature and high-pressure conditions. The results show that the increase of carbon dioxide content in natural gas will cause the total energy of natural gas molecules to decrease when the pressure is constant. This means that the higher the CO 2 content in natural gas, the easier it will be compressed. This study should lay the foundation for investigating the mechanisms of the occurrence of CO 2 -containing natural gas, as well as facilitating the exploitation of CO 2 -containing natural gas., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)