The reservoir sedimentation is an important problem in reservoir projects. Because thesedimentation volume of reservoir is influenced by many indefinite factors, such as inflow,sediment and reservoiroperation. Actually, there is the risk that sedimentation volume exceeds the design standard.In this paper, inflowand sediment are regarded as main indefinite factors. The risk can be analysed by randomly simulating inflow andsediment series and calculating reservoir sedimentation,which will be taken as the basis of the division-making.