This paper discusses the problem of the way to determine the value of beginning rain causing debris flow when sufficient observed rainfall data for the gully concerned are not available,The method of fuzzy similarity selection is applied for this purpose by making gullies as models for comparison whose value use of some debris flow of beginning rain debris flow are known. For any debris flow gully, the possible range value of beginning rain can be determined by analysing the degree causing of the of similarity models,the range of which would likely be the dangerous enveloped area of rainfall condition of would likely be the dangerous that gully.