Based on the daily meteorological data in the recent 40-year and the flow-off data for more than one hundred,the climate features during the damming period in Sanxia area and the variabilities of the weather element,such as temperature,precipitation,wind and weather phenomenon,which would affect the engineering procedures,are analysed in detail.The statistical relationship between the streamflow at dam site during the damming period and precipitation for previous period in the upper reserivor area,and then the longterm forecast model of streamflow are established.