Abstract: Reasonable rainfall threshold index is not only related to historical rainfall, but also affected by the regional climate, landform, geological deposition, vegetation, man-made activities and so on. These factors may fall into two categories: rainfall and environmental geological background. The method of definition of rainfall event can be used to calculate rainfall, which includes three aspects: antecedent precipitation, active rainfall and the maximum rainfall in one hour. The methods of RS and GIS are used in the assessment of the susceptibility of debris flows, concerning the slope, rock soil mass types, man-made activities, geological structure, and vegetation, etc. Using rainfall data collected during the occurrence of 82 debris flows from 1949, this paper puts forward a method of the rainfall thresholds obtained for various assessment of the susceptibility of debris flows. The results are used in the Xiqu Gully in Fangshan district of Beijing on July 17, 2015, illustrating that this method is reasonable and feasible. This paper analyzes the regional geological environment background and provides a new thread for the debris flow forewarning in mountain areas.