In the present study, the three-dimensional steady-state numerical simulation has been performed by using ANSYS Fluent15.0 with SST k-ωlow Reynolds turbulence model to study flow and heat transfer characteristics for supercritical CO ₂in 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 CO ₂in horizontal tube. Firstly, flow and heat transfer characteristics of supercritical CO ₂was studied in horizontal tube. Based on the assumption that the supercritical CO ₂will 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 CO ₂in horizontal tube are similar to those under subcritical pressure. Then, the influences of heat flux q _wand mass flux Gon flow and heat transfer of supercritical CO ₂were analyzed. The higher heat flux q _wis or the smaller mass flux Gis, the higher inner wall temperature T _w,iat top generatrix is. The reasons for difference in the distribution of inner wall temperature T _w,iat top generatrix under different heat flux q _wand mass flux Gwere explained by capturing detailed information about thermophysical properties distribution including specific heat at constant pressure c _pand 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 _pand thermal conductivity λ, axial velocity uand turbulent kinetic energy kare 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 CO ₂Brayton cycle.