Abstract: Supercritical carbon dioxide (S-CO₂) is an excellent working medium in the Breton cycle and is widely used in the natural circulation and forced circulation systems of the fourth-generation reactors. To clarify the differences between forced circulation and natural circulation in heat exchange, as well as the differences between horizontal flow and vertical upward flow in heat exchange modes, this paper comparatively analyzed forced circulation and natural circulation in flow heat transfer through theoretical and numerical simulation. Based on the changes in the buoyancy force and the expansion acceleration effect, the changes in the heat exchange mode during the horizontal flow and the vertical upward flow were theoretically analyzed. Finally, numerical simulation was conducted to study the forced circulation flow heat transfer characteristics and radial thermal stratification characteristics of S-CO₂ under different inclination angles and pressures. The research results show that the flow heat transfer characteristics of natural circulation and forced circulation are consistent under the same geometric structure, initial flow parameters, and heating conditions. The direction of the buoyancy force leads to the non-uniform heat transfer phenomenon under horizontal flow. The local heat transfer deterioration caused by buoyancy force in horizontal flow is more severe than that in vertically upward flow. The variation of inclination angle from 0° to 10° is the range that has the greatest impact on heat exchange deterioration, increasing the heat exchange efficiency by 65.6%. As the inclination angle increases, the mode of heat exchange deterioration changes from the horizontal deterioration mode to the vertical deterioration mode. Under normal heat exchange mode, the influence of an inclination angle of 80° to 90° on radial thermal stratification is 3 to 6 times that of a linear increase from 0° to 80°. In the heat exchange deterioration mode, the variations of inclination angles from 0° to 10° and 80° to 90° have the greatest impact on the radial thermal stratification behavior.