The invention discloses a method for determining the cascade reservoir interval incoming sediment amount and distributing the cascade reservoir interval incoming sediment amount along the flowing path. The method includes the steps that firstly, the sediment runoff data of cascade reservoir main stem and branch control stations, damsites and intervals are collected; secondly, the sediment runoff and process of the damsites in the typical series year are determined according to the ratio of the average yearly sediment runoffs of the damsites to nearby control hydrometric stations in multiple years; thirdly, the sediment runoff difference between the upstream reservoir damsite and the entering-reservoir control station serves as the uncontrolled interval incoming sediment amount, and the sediment runoff difference between the upstream reservoir damsite and the downstream damsite serves as the uncontrolled interval incoming sediment amount of the downstream reservoir; fourthly, in the uncontrolled interval, when one branch has no sediment runoff data in the typical series year but has the average yearly sediment runoff data in multiple years, the branch uses the average yearly sediment runoff data in multiple years; and fifthly, other incoming sediment amount except for the know branch in the uncontrolled interval is evenly distributed into the main stem watercourse along the flowing path. By means of the method for determining the cascade reservoir interval incoming sediment amount and distributing the cascade reservoir interval incoming sediment amount along the flowing path, the cascade reservoir interval incoming sediment amount can be determined reasonably, and technical support is provided for prediction and calculation of sediment erosion and deposition of the cascade reservoir.