After analysing the data of typical investigation and hydrological survey in Northwestern Shanxi province,the result shows that the calculating result would fit to the basic characteristic of storing more flood and sediment by silt arrester in the year of more flood and sediment than mormal year if we calculate the amount of annual flood and sediment reducing.A new method of calculating the sediment retaining in middle reaches of the Yellow river is suggested,it has used in the analyses of sediment and flood reducing benefits on the eight tributaries of Yellow river in northwestern Shanxi province,and a good result has been obtained.