Abstract: In order to determine the optimal process parameters for fire prevention with continuous nitrogen injection at 5-2S-2 fully mechanized caving goaf. The physical-mathematical model of goaf flow field was established on the basis of the basic theory of computational fluid mechanics. Numerical simulation of goaf oxygen concentration distribution and "three zone" division under different nitrogen injection condition was conducted by using COMSOL software. The optimal parameters of nitrogen injection are as follow: 400 m³/h injection capacity of nitrogen, nitrogen injection port position away from the face of 30 m, the nitrogen injection time of 15 h/d.