Abstract: Underground coal gasification extracts underground coal seams in the form of natural gas, which is a clean and efficient way of using energy. Ignition is the first step to start the gasification reaction, and it is necessary to analyze the ignition and combustion characteristics of fuels during chemical ignition. N-heptane is selected as the ignition fuel, and the Discrete Phase Model is used to simulate the atomization process of n-heptane. The co-flow combustion model of heptane droplets and oxygen is established. The ignition temperature and combustion flame characteristics of heptane are explored, and the key factors affecting the ignition process of underground coal gasification are analyzed. The results show that with the increase of pressure, the ignition temperature of heptane is lower, and it reaches 670 K at 10 MPa. When the droplet particle size is in the range of 0.1–0.4 mm, the ignition temperature increases slightly with the decrease of particle size. Increasing the oxygen concentration in the oxidant can significantly increase the flame temperature, but it would lead to a shorter flame length. In order to avoid the melting problem of coal, it is more appropriate to control the oxygen concentration around 20%~30%, and the maximum temperature of the wall surface is within 1 200~1 530 K. The flame temperature is lower during lean combustion, but when the excess air coefficient is 0.6~0.8, the heating temperature and heating length of the wall surface are higher than that of oxy-fuel combustion, which can make more efficient use of fuel in the process of ignition.