Abstract: Throughstatistical analysis ofdebrisprovenancereserves, effective catchment area, rainfalland otherdata in Beijing, critical rainfallinducing debris flowis proposed in the calculation formula. In the formula, critical rainfall is proportionalto effective provenancereserves in the ditch, and is inversely proportional tothe effective catchment area. After applying the formula to the cases which happened previously, the critical rainfall is calculated using back analysis method according to the record the amount of accumulation of debris. At last, the critical rainfall is compared with the actual rain records. The results are showed that the critical rainfall can predict correctly the debris flow in different geological conditions. It is a good technical support for prevention of debris flow to reduce the property loss in the Beijing Area.