Abstract: To elucidate the mechanisms of reignition and thermal equilibrium disruption in sealed underground coal fires, three coal samples of different ranks (anthracite, bituminous coal, and lignite) were selected. Physical simulation fire extinguishing experiments using 20 kg- 80 kg coal samples,as well as inert -gas injection and reignition cycling experiments using 1. 5 t coal samples,were conducted. The results show that the temperature decay in a sealed fire zone exhibits significant nonlinear characteristics,with approximately 400 ℃ serving as the demarcation point. The temperature decreases rapidly in the high-temperature stage and slowly in the low-temperature stage. The greater the coal sample mass, the longer the natural extinguishing time. The reignition cycling experiments involved multiple temperature nodes within the range of 800 ℃ to 200 ℃. By alternately injecting inert gases (N₂ and CO₂) and resuming air supply, it was found that CO₂ exhibits superior fire suppression efficiency to N₂. The critical reignition temperature for the three coal samples ranges between 250 ℃ and 300 ℃. Below this temperature threshold,the heat-production-heat-dissipation balance of the coal mass is disrupted,and even restoring the oxygen supply cannot reverse the cooling trend. Based on Fourier’ s law of heat conduction and the principle of energy conservation,a cylindrical reactor was adopted as the physical model. Considering the thermal resistances and heat exchange among the coal mass,reactor wall,and airflow,a mathematical model describing the temperature decay in a sealed fire zone was established. A prediction formula for the theoretical minimum time required for natural extinguishing was derived. The model shows good agreement with experimental validation and can provide a theoretical basis for scientifically determining the timing of unsealing sealed fire zones. The results indicate that relying solely on indicator gas concentrations is insufficient for accurately assessing reignition risks;a comprehensive evaluation system incorporating temperature field evolution should be established.