Carbon capture, transport and storage (CCS) plays an important role in the process of carbon neutralization. The flow and heat transfer process of supercritical CO₂ and supercritical CO₂ mixture in a vertical circular tube was studied with supercritical CO₂, supercritical CO₂+CH₄ mixture (CH₄ mole fraction is 1%, 3%, 5%) and supercritical CO₂+N₂ mixture (N₂ mole fraction is 1%, 3%, 5%) as working fluids at a mass flow rate of 300～600 kg/(m²?s), heat flow density of 80～100 kW/m², and inlet pressure of 8～10 MPa. The results show that with the decrease of CH₄ and N₂ mole fraction in the working medium, the peak value of heat transfer coefficient of the working medium increases gradually, and its corresponding temperature increases gradually; the heat transfer coefficient of the working fluid increases with the increase of the mass flow rate, and the greater the mass flow rate, the greater the change range of the heat transfer coefficient; with the increase of inlet pressure, the peak value of heat transfer coefficient of working fluid decreases, and its corresponding temperature increases gradually. Turbulent kinetic energy, buoyancy and specific heat capacity at constant pressure are closely related to the heat transfer enhancement of supercritical CO₂ and its mixture.