High-temperature flat heat pipe (HTFHP) is a highly effective heat transfer device in spacecraft thermal control. In this paper, a high-temperature flat heat pipe equalizer (200 × 100 × 30 mm) is designed innovatively, and the metal fiber felt is used as the capillary wick of the HTFHP, and sodium is filled into the compact cavity as the working medium. The effects of heating power and inclination on the temperature uniformity of the HTFHP were experimentally studied. The results show that HTFHP with sodium as working medium has fast start-up characteristics and good thermal uniformity, and can adapt to various variable inclination conditions. The heat transfer coefficient of this HTFHP can reach 498.702 W/m2K when the heat input is 600 W, and the time for the heat pipe to reach a steady-state is 2020 s. In addition, the temperature difference showed a significant decrease from 80 °C to 50 °C when the heat loss from the center to the edge is from 4.2 % to 3.7 %. The inclination has little effect on the temperature distribution of the HTFHP, and the temperature uniformity is better when the inclination angle is 15°. When the heat input is 600 W, the heat loss from the center to the left and right edges is the smallest, which are 5.5 % and 6.2 % respectively. Overall, this HTFHP has good thermal performance and is worthy of further optimization.