In the present study, cooling heat transfer of supercritical CO2 in a horizontal helically coiled-tube 4 mm in diameter, 2000 mm in length, a pitch of 10 mm and 0.1 in curvature is numerically investigated with RNG turbulence model. Influences of mass flow rate, heat flux and pressure on heat transfer of supercritical CO2 are studied. The characteristics of the flow and heat transfer are compared with those in a horizontal straight tube. Results show that the secondary flow and heat transfer coefficient in a helically coiled tube is obviously larger than in a horizontal straight tube. The heat transfer coefficient of supercritical CO2 increases with increasing mass flow rate, and the heat transfer coefficient increases slightly as the heat flux increases in the gas-like region, while the heat transfer coefficient is unaffected by heat flux in the liquid-like region. The peak of the heat transfer coefficient decreases and shifts to a higher temperature region as the pressure increases.