Underground coal gasification (UCG) has its own reaction characteristics with the growth of the combustion cavity because the coal seam is fixed while the gasification face moves. The theoretical understanding of UCG requires continuous and deep research. In order to explain the UCG mechanism by means of simulation calculation, based on the analysis of the reaction process of UCG, radial and axial reaction models are established. The comprehensive heat transfer coefficient of the coal seam is determined by comparing the calculation results with the field test results. The differential method is used to calculate variations in the composition of the outlet gas, the temperature field distribution in the channel, the thickness of the radial reaction zone, and the influence of the oxygen fraction on the length of the oxidation zone. The results indicate that in one gasification cycle, the changes of gas composition is close to that of the field test, which initially increases and then decreases with increasing gasification time. The radial expansion of the coal seam mainly occurs in the high temperature zone, consistent with the trend of the channel temperature field. The radial expansion thickness in the coal seam is between 0.2 and 0.8 m. The higher the oxygen fraction in the gasifying agent is, the shorter the range of the oxygen distribution along the axial direction will be, that is, the shorter the length of the oxidation zone.