To validate the thermal–hydraulic software model for the prismatic graphite components of a High-Temperature Gas-cooled Reactor (HTGR), a comprehensive thermal–hydraulic study was conducted using a full-scale, single-column prismatic graphite component and a helium-cooled thermal–hydraulic test loop. A thermal test matrix was established under steady-state high-temperature and high-pressure conditions for the prismatic graphite components. The influence of the gap between the heating elements on heat transfer was analyzed, and temperature data for the heating elements, graphite, and helium were collected. The results indicated that, except for the inlet section, the temperature distributions of all three groups were approximately linear along the axial direction. The helium temperature was the lowest, with a temperature difference of about 50 °C between the helium and the heating elements along the axis. The temperature of the graphite component showed minimal difference compared to that of the heating elements, suggesting good thermal contact between the heating elements and the graphite components. Theoretical calculations showed that as the heating power increased, the gap thickness between the heating elements and the graphite components gradually decreased along the axial direction, improving heat conduction efficiency. The effect of the gap thickness on thermal conductivity was found to be limited. A comparison between the experimental graphite component temperatures and the numerical simulation results showed good agreement. Additionally, helium flow resistance coefficients were calculated at different Reynolds numbers using experimental flow data. Flow resistance characteristics were then fitted under conditions of equal flow rate and varying heating power, and a formula was derived. This study provides important guidance for the design optimization and model validation of prismatic HTGR cores.