By statistic analysis of surveyed data of precipitation,the flood and sediment of Sanchuan river basin,the mechanism of sediment transportation is probed,and a mathematical model of precipi tation-floodsediment transportation is set up through regression analysis.This model has been successfully used to calculate the sediment and the flood reducing benefits by soil and water conservation measures in Sanchuan river basin from 1970 to 1996,the sediment and flood variation in 1990s are analyzed emphatically.Another four kinds of hydro-ana lysis methods are also been used to verify the results.A serialization method for processing precipitation data and inclination of runoff coefficient variation as w ell as the variation of riverbed erosion and deposition are also analyzed.This method can be used for reference to other branches of the middle reaches of Yellow river with rich and coarse sediment.