Abstract: In this paper the heat transfer characteristics of high temperature flat heat pipe (HTFHP) are theoretically and experimentally investigated using sodium metal as the working medium of HTFHP. The experimental study and result analysis on the heat transfer characteristics of the heat pipe under the conditions of different inclination angles and different heating powers show that the HTFHP has a very good temperature uniformity performance under different inclination angles, and increasing the inclination angle can accelerate the reflux of the liquid work material, promote the reflux of the liquid to the condensing section, and enhance the temperature uniformity performance of the heat pipe. The overall temperature of the heat pipe appears to decrease and then increase with the increase of the inclination angle. With the increase of heating power, the heat transfer characteristics of the HTFHP is better, and the increase of heating power can promote the evaporation and condensation cycle of the working medium inside the heat pipe, and strengthen the heat transfer characteristics of the heat pipe. Moreover, the equivalent thermal conductivity numerical method is used based on the computational fluid dynamics, and through the comparison of the simulation results with the measurement points experimental data, the error of the inclination angle is within 6.7%, and the error of the heating power is within 6.53%.