In order to enhance the heat transfer performance of CO₂ heat pump, the heat transfer performance of supercritical carbon dioxide in horizontal straight tube is conducted based on Fluent numerical simulation method. Cooling heat transfer of supercritical CO₂ in a horizontal tube with diameter of 4 mm and length of 2 000 mm is numerically investigated with k-ε turbulence model. The temperature field and heat transfer coefficient of supercritical carbon dioxide fluid under in-tube cooling conditions are mainly investigated. The effects of mass flow rate and inlet temperature on heat transfer performance in horizontal straight tubes are studied. 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 100 kg/(m²·s²), and the average heat transfer coefficient increases by about 12%. With the increase of supercritical carbon dioxide inlet temperature, the average heat transfer coefficient becomes small, but the maximum value of the local heat transfer coefficient doesn’t change, only delaying the appearance of the maximum value of the local heat transfer coefficient. The research result may provide important theory and data support for the application of the horizontal straight tube in CO₂ heat pump.