Abstract: The debris flows with low occurrence frequency are of more harmfulness than the debris flows of high occurrence frequency. This paper, on the basis of field investigation and material analysis about the typical low-frequency debris flows occurred in recent years, discusses the characteristics of low frequency debris flows. In order to further discuss the forecasting method of low frequency debris flows, the grey theory is applied and a grey catastrophe forecasting model was established. The Chayuan gully in Wenchuan county, Sichuan province is used as an example. The paper puts forward the corresponding prevention advices for low frequency debris flows according to the their characteristics. This discussion is of certain significance for debris flow subject in theory and for the hazard prevention and mitigation in mountain areas in practice.