Abstract: Most debris flows in the mountainous areas of Beijing belong to channel erosion type, which have characteristics as low frequency, long intermittent cycle and massive developing.Without the support of abundant debris flow events, early warning of debris flow has always been a problem.In this paper, firstly, the accuracy of existing warning models for Beijing area were verified.Then, based on years of monitoring rain data, process cumulative rainfall(R) and maximum hourly rainfall intensity(I) were considered as two-dimensional discrete random variables, the joint probability distribution characteristics of rain fields were analyzed, and the probability line of rainfall safety of debris flow-free events were established.Finally, the rainfall safety probability 4% curve was set as the rainstorm debris flow early warning curve in Beijing mountain area based on a small number of debris flow samples.The results show that: when the process cumulative rainfall(R) reaches 220 mm and the maximum hourly rainfall intensity(I) is greater than 60 mm/h, the possibility of debris flow occurrence is greater.The proposed early warning curve can meet the needs of debris flow early warning with present geological conditions and provides a technical reference for the establishment of early warning models in areas without abundant debris flow events.