River runoff series has the characteristics of fractal and gray. Therefore, by combining gray system theory with R/S analysis, R/S gray prediction model was proposed to forecast annual runoff. Based on the data of the annual runoff at the Zhengyixia Station of the Heihe River during the period 1949 -2011, the runoff series were analyzed by R/S method. The Hurst exponent and average cycle of river runoff series were determined firstly; then, in one cycle, annual runoff was predicted by gray. The prediction results show that R/S gray prediction model has higher precision indeed. This method broadens the application range of fractal theory and grey model, and provides a new scientific approach for predicting annual runoff