Abstract: This paper discusses the interference of forest fire in forest ecosystem proposes to use the forest fire destructive force which called FOREST FIRE SEVERITY INDEX (FFSI) as a measuring standard and also suggests to use the volume of forest (timber) as the index of the state of its forest ecosystem. According to the physical analysis of the process of the spreading of forest fire, a combustion spreading equation is deduced as a model and the destructive force of the fire of the Xingan-mountain larches is calculated at different spreading speeds. On the basis of the investigation results of the consumptive available fuels, the authors point out how forest fire influences forest ecosystem at different levels of FFSI and how forest fire should be dealt with according to its intensity.