In the present study, the three-dimensional steady-state numerical simulation has been performed by using ANSYS Fluent15.0 with SST k-ω low Reynolds turbulence model to study flow and heat transfer characteristics for supercritical CO₂ in the horizontal straight tube with inner diameter d_i = 22.14 mm and heating length L_h = 2440 mm under heating condition. The reliability and accuracy of the numerical model was verified by the experimental data of flow and heat transfer of supercritical CO₂ in horizontal tube. Firstly, flow and heat transfer characteristics of supercritical CO₂ was studied in horizontal tube. Based on the assumption that the supercritical CO₂ will undergoes “phase transition” between liquid-like and vapor-like at pseudocritical temperature T_pc, the differences between top generatrix and bottom generatrix of horizontal tube at flow and heat transfer behaviors were revealed. The results show flow and heat transfer characteristics of supercritical CO₂ in horizontal tube are similar to those under subcritical pressure. Then, the influences of heat flux q_w and mass flux G on flow and heat transfer of supercritical CO₂ were analyzed. The higher heat flux q_w is or the smaller mass flux G is, the higher inner wall temperature T_w,i at top generatrix is. The reasons for difference in the distribution of inner wall temperature T_w,i at top generatrix under different heat flux q_w and mass flux G were explained by capturing detailed information about thermophysical properties distribution including specific heat at constant pressure c_p and thermal conductivity λ, axial velocity distribution and turbulent kinetic energy distribution in the fluid domain. It is observed that vapor-like film thickness δ, vapor-like film property characterized by specific heat at constant pressure c_p and thermal conductivity λ, axial velocity u and turbulent kinetic energy k are the main factors affecting the difference in inner wall temperature distribution at top generatrix. The present work can provide a theoretical guidance for design and safe operation of heat exchanger for supercritical CO₂ Brayton cycle.