Abstract: This paper introduces in detail the thought and way of long term forecast of water regime of reservoirs,based on the method of step regression and optimization of predictors,to reduce the error of prediction to the minimum.The way was effective for long term forecast of water regime of several reservoirs in the past.Forecast for the three hydraulic plants in the North East region of China is illustrated in detail and flow prediction for the June-September flood period in this region is part icularly suggested in this paper.We think that this method is practical for long termprediction for water regime of reservoir.