Abstract: Underground coal fire is a kind of global major disaster, which can maintain long-term self-sustained smoldering under the thermal buoyancy driven by the high-temperature flue gas generated within the fire. This self-sustaining characteristic is the fundamental reason for the persistent, disastrous, and highly challenging nature of underground coal fire control. However, understanding the mechanism behind this self-sustaining behavior remains an unresolved scientific challenge. This study aims to explore the principle of self-sustaining characteristics of underground coal fires from the perspective of energy analysis. A small-scale experimental device is set up to carry out physical similarity experiments of underground coal fires driven by thermal buoyancy. Porous foams with different thicknesses are used to construct different thermal buoyancy driven airflow conditions. Combined with numerical simulations, the study examines variations in flus gas temperature, thermal buoyancy, and the resulting airflow under different smoldering conditions. Furthermore, the relationship between oxygen supply energy consumption driven by thermal buoyancy and the energy of smoke exhaust is analyzed. The results show that with the increase of foam thickness, the flus gas temperature gradully decreases, thermal buoyancy and the wind speed caused by it decrease, the smoldering intensity of underground coal fire decreases, and the energy consumption driven by thermal buoyancy decreases, but the ratio of thermal buoyancy to the exhaust energy of flus gas changes little. The ratio of oxygen supply energy to flue gas emission energy of thermal buoyancy drive increases with increasing crack depth, but the overall ratio is very small, and the change of crack depth varies within the range of 10⁻⁵-10⁻³. Research shows that compared with flue gas discharge energy, only a very small proportion (as low as one in a hundred-thousand) can be used to realize the self-sustaining spread of underground coal fires driven by thermal buoyancy, which is one of the fundamental reasons why underground coal fires are extremely difficult to control.