The heat transfer performance of supercritical carbon dioxide in horizontal straight tube is conducted based on Fluent numerical simulation method. The temperature field and heat transfer coefficient of supercriticalcarbon dioxide fluid under in-tube cooling conditions are mainly investigated. The effects of mass flow rate and inlet temperature on heat transfer performance are studied for horizontal straight tubes. The simulation results show that the heat transfer coefficient of supercritical carbon dioxide increases with the increase of mass flow, the mass flow increases by 100kg/m_².s, and the average heat transfer coefficient increases by about 12%. With the increase of supercritical carbon dioxideinlet temperature, the inside of the tube The average heat transfer coefficient becomes small, but does not affect the maximum value of the local heat transfer coefficient, and only delays the maximum value of the local heat transfer coefficient.