Abstract: Spontaneous combustion of residual coal in goaf is one of the main accidents affecting coal production safety. Based on Fluent numerical simulation technology, a three-dimensional physical model of goaf was built, the three-dimensional distribution law of the flow field in goaf was simulated and studied, and the dangerous areas of the spontaneous combustion of residual coal in goaf were divided by the O₂ volume fraction as the discriminant index; by analyzing the spatial distribution of residual coal in goaf and inert gas characteristics of different inert gases, the compound inerting technology in fully mechanized working face in goaf was put forward, namely high pressure injection of CO₂ and low pressure injection of N₂, and it was compared with the single gas inerting technology. The results show that the compound inerting technology has better fire prevention and extinguishing effect than the single gas inerting technology; in engineering application, the CO volume fraction at each monitoring point is reduced to less than 2.4×10^-5 in a short time, and the fire prevention and extinguishing effect is good during injection of inert gas.