Since supercritical carbon dioxide (S-CO2) systems usually have to work at both high temperature, high pressure and high heat flux, selecting appropriate solid materials is of great important to their system safety. In this study, the thermofluidic characteristics of supercritical carbon dioxide (S-CO2) in a horizontal rectangular channel have been investigated under high pressure and one-side-wall heated with high heat flux. Four different solid wall materials (253 MA, Inconel 617, Haynes 230 and Haynes 233) and three different heat flux values (1.5 MW/m2, 2.0 MW/m2 and 2.5 MW/m2) are selected for analyzing the impacts of wall material and heat flux boundary conditions. The results showed that the maximum wall temperature difference of all four wall materials can generally exceed 100 K under the minimum heat flux, and can reach 500 K for Haynes 233 at the heat flux of 2.5 MW/m2. Considering the maximum allowable stress and creep characteristics, Inconel 617 has more obvious advantages as a solid material at the heat flux below 2 MW/m2, while Haynes 230 is a better choice at the heat flux beyond 2 MW/m2 because of the stronger mechanical properties. By exploring the effect of inlet temperature, it is found that the inlet temperature close to the pseudo-critical temperature is conducive to flow and heat transfer. Taking the effect of buoyancy into account, it is shown that the temperature of the heating surface is decreased, the deterioration of heat transfer is weakened and occurs early, and the difference on the cross sections of the wall temperature decreases.