Abstract: In this paper,based on the former studies and considering the propability theory and mechanical analysis method,as well as shelter-exposure effects,a pattern for calculating the probability of incipient motion in non-uniform bed material is established.And a simple model found on the pattern for multi-step predicting the process of scouring and armoring is proposed here in,it should be a useful tool for forecasting the armoring-layer size distribution and scouring depth,while uses many observed data to prove it and yields satisfactory results.