Debris flow is a common disaster in mountain regions. The valley slope, stormrainfall and amassed sand-rock materials in a watershed may influence the types ofdebris flow. The bursting of debris flow is not a pure random event. Field investigationsshow the periodicity of its burst, but no directive evidence has been found yet. A riskdefinition of debris flow is proposed here based upon the accumulation and the startingconditions of loose material in channel. According to this definition, the risk of debris flowis of quasi-periodicity. A formula of risk estimation is derived. Analysis of relative factors reveals the relationship between frequency and size of debris flow. For a debris flowcreek, the longer the time interval between two occurrences of debris flows is, the biggerthe bursting event will be. Debris flow is a common disaster in mountain regions. The valley slope, stormrainfall and amassed sand-rock materials in a watershed may influence the types of debris flow. The bursting of debris flow is not a pure random event. Field investigationsshow the periodicity of its burst , but no directive evidence has been found yet. A riskdefinition of debris flow is proposed here based upon the accumulation and the starting conditions of loose material in channel. According to this definition, the risk of debris flowis of quasi-periodicity. A formula of risk Analysis of relative factors reveals the relationship between frequency and size of debris flows. For a debris flowcreek, the longer the interval between two occurrences of debris flows is, the biggerthe bursting event will be.