In this paper, the observational researches and results of fault gases at fixed points along some active faults since 1986 are introduced. The dynamic precursory variation of fault gases is observed before 5 earthquakes (Ms≥4.5)occurred in west Gansu Province since 1987. The dynamic precursors of fault gases before the earthquakes have following characteristics: variation range is wide; precursors immediately before events are prominent; the components of many kinds of gases change simultaneously, etc. For an about M=5 earthquake, its precursory range may reach 500 km. The authors consider that the method is suitable for monitoring immediately before event in seismic risk areas.