Abstract: Underground coal gasification (UCG) as an alternative coal mining technology is attracting much attention. However, due to the high experimental cost of UCG, the evolution mechanism of coal seam should be deeply explored prior to the implement of experiments. By virtue of the great computational power of computers, the computational fluid dynamics (CFD) is able to simulate the production process of UCG at a relatively lower cost. The in-situ ignition and combustion of deep coal seam heated by 1 000 K flue gas is simulated and the changes of temperature, molar O₂ fraction and porosity are analyzed by the CFD software, Fluent, developed by Ansys cor-poration. It is elucidated that pumping 1 000 K flue gas for 500 s is inadequate for coal seam ignition, while 1 000 s heating has been enough for coal seam ignition. On the internal surface of coal seam, the high temperature is accumulated in the 0−0.6 m coal seam near inlet. The maximum temperature before ignition is below inlet temperature of heating flue gas, 1 000 K, while the maximum temperature after ignition exceeds 1 000 K and reaches up to 1 250 K. Almost all O₂ is consumed in the high-temperature zone, while less than 2% molar O₂ fraction arrives at the internal surface of coal seam in other lower-temperature zones. The porosity rapidly increases in high-temperature zone and at 1000 s the porosity in the part of coal seam reaches up to 0.9. In the interior of coal seam, the thicker the coal seam is, the lower the temperature rises. The maximum temperature on 7 cm line is only 500 K at 1 000 s and 2 000 s, much lower than that on the internal surface of coal seam when it is heated for the same time. During 2 000 s ignition, little O₂ penetrates 7 cm and 14 cm coal seam and the porosities of interior coal seam are all below 0.4. The low porosities are attributed to the low temperature and low molar O₂ fraction. The simulation on temperature, molar O₂ fraction and porosity may provide a reference for how to promote the reaction in the interior of coal seam.