Abstract: At present, the current rainfall threshold of debris flow in Beijing are mostly collected from the historical rainfall data of debris flow, lacking the analysis that combines with the debris flow mechanism and rainfall characteristics.By searching relevant document, this paper analyzed the debris flow in the mountainous areas of Beijing and calculated the rainfall threshold triggering debris flow according to their failure mechanism and the corresponding debris flow forecasting model.The results show that:(1) heavy rainfall is closely related to debris flow in the study area, and most of them are short duration rainfall.(2) the early warning range of debris flow in the mountainous areas of Beijing is as follow: when the 1h triggering rainfall is I≥42 mm and the previous accumulated rainfall is B≥130 mm, it is highly likely to cause debris flow; when the 1h triggering rainfall is 30≤I<42 mm and the previous accumulated rainfall is 75≤B<130mm, the likelihood of debris flow is moderate; when 1h triggering rainfall is I<30mm, and the previous accumulated rainfall is B<75mm, it is less likely to cause debris flow.(3) Based on the integrated rainfall threshold R*=12.51+B,the red alert for heavy rainfall in mountainous areas of Beijing is R*≥600mm, orange alert for heavy rainfall is 600>R*≥480mm, yellow alert for heavy rainfall is 480>R*≥300mm, and blue alert for heavy rainfall is R*<300mm in the mountainous areas of Beijing.The study can provide a basis for debris flow prevention and mitigation.