Abstract: Sediment motion has both definite respect and stochastic respect.in this paper,the stochastic feature and expression of flowing water,size of bed meterial and sediment motion have been analyzed.It is pointed out that there are four basic forms of sediment motion in flowing water,and the four basic forms of sediment motion usually transter each other with the variation of flowing water and sediment conditions,and a probability exists from a form of sediment motion to other form,so there are sixteen probability for four forms of sdiment motion.The sixteen transfer probability can be represented by the four basic probability of the four basic forms.The calculated methods of the four basic probability have been studied by theoretical analysis.The measured data agree with calculated basic probability of the four basic forms very well.