Using grey theory with the data of annual curvy amount of water carry silt which gained from 1952 to 1985 for total 34 years at the emit mouth station (Zhang jiashan Station) of Jinghe watershed,the author establisped a group of grey models to analyze the ten years'benefit of decreasing silt during 1976 and 1985,This group of models was proved effect by checking the results with hydrological method,It presents a new method for calculating the benefit of decreasing silt and provide the basis for forecasting the soil and water loss in comprehensive harness in large values.