In order to explore the intrinsic mechanism of heat transfer deterioration (HTD) in supercritical fluids, the flow and heat transfer characteristics of supercritical CO2 in a uniformly heated vertical circular tube were investigated by experiments and numerical simulations. The inner diameter of the circular tube was 14.5 mm, and the range of experimental parameters covered a pressure of 7.5–9.5 MPa, a mass flux of 320–940 kg/(m2s), and a heat flux of 17–160 kW/m2. The effects of heat flux, mass flux, and inlet temperature on the heat transfer characteristics of supercritical CO2 were investigated. Results reveal that an increase in heat flux causes an intensification of HTD and a forward shift of the peak wall temperature towards the inlet. The increase in mass flux can weaken the HTD and enhance the overall heat transfer coefficient, but it may result in heightened localized HTD. As the inlet temperature increases, the overall heat transfer coefficient improves. The first wall temperature peak shows an increasing and then decreasing trend, while the second wall temperature peak gradually decreases with the increase in inlet temperature until it disappears completely. Numerical simulation studies were carried out using a modified variable Prt turbulence model to analyze the cause of HTD in supercritical CO2. The deterioration in heat transfer was found to be closely related to the violent thermophysical changes of supercritical fluids. The drastic physical property variation results in the buoyancy force effect, which alters fluid shear stress distribution and turbulent kinetic energy generation and thus leads to HTD.