Abstract: This paper analyses the coaction mechanism between scouring and armoring. Based on the research of stochastic distribution characterstics of exposing height for nonuniform sediment, the velocity profile.and the law of threshold motion, the stochastic model of scouring and armoring for riverbed is put forward. The model can calculate scour depth of riverbed and armoring steady graduation simultaneously. The testing and verifying result show that the calculating results are corresponded with observed values on the whole.