The Li/SF6 combustion reaction can provide power for unmanned underwater vehicle (UUV) due to the advantages of high heat release and no gaseous combustion products. Exploring the combustion and heat transfer characteristics of Li/SF6 can greatly improve the performance of the UUV combustor. In this paper, based on the operation requirements of UUV combustor, the C300 combustion reactor is designed, the Li/SF6 combustion experimental system is established, and the Li/SF6 combustion and heat transfer experiments are performed. The experimental results show that the overall combustion process lasts about 1.55 h, and the flame temperature is higher than 1768 °C. The temperature in the gas phase decreased gradually along the radial direction and increased along the axis. Eight quasi-equilibrium states are detected, and the maximum combustion power is 12.73 kW. As the pressure increases, the combustion power will increase accordingly. Under the maximum power, the heat transfer efficiency of the C300 combustion reactor can be calculated to be by using water as the heat transfer medium. The heat transfer quantity of the combustion furnace Qf is 2.07 times that of the combustion chamber Qc, and the heat generated by combustion is concentrated in the lower part of the combustion reactor. In addition, a thermal resistance model is proposed to optimize the heat transfer process. Finally, the upper fuel is fully combusted, while the bottom fuel is poorly combusted due to the phenomenon of insufficient melting and combustion product covering.