Abstract: The paper analyses the trend of annual runoff series for Guanting Reservoir of Yongding River and Fenhe Reservoir of Fenhe River. The meteorological factors and human activities resulting in the trend are studied. The research results show that the main factors affecting the trend of annual runoff are the trend of precipitation corresponding fluctuation of precipitation during the some periods and the increasing of agricultural.industrial,forestrial and domestic water use in the basin. According to historical data and possible human activities of the basin under study in the future,the predicted results of long trem annual runoff are presented.