Abstract: The FLUENT software was used to numerically simulate the heat transfer characteristics of CO₂ in a horizontal tube with the diameter of 4 mm and the length of 1 000 mm under supercritical pressure. First, the effect of heat flux on heat transfer was analyzed, indicating that under the condition of mass flow of 140 kg/（m~2·s）, the top and bottom walls presented a large temperature difference and showed the abnormal characteristics of heat transfer behaviour. Then two kinds of buoyancy criteria were compared and evaluated, and the effect of pressure on buoyancy was analyzed. Finally, the heat transfer characteristics of the horizontal tube walls were revealed via a study on the distributions of profiles’ physical properties. Results show that the larger the heat fluxes are, the more obvious the deterioration of heat transfer and the stronger the buoyancy effect will be achieved. The B_P criterion can better predict the buoyancy distribution under above conditions. Besides, the wall temperature difference can reflect the strength of buoyancy. Increasing pressure can mitigate the changes of CO₂ physical properties and decrease the effect of buoyancy.