In this paper introduced Artificial Neural Networks(ANN),which is different from tradition way and has been used to take out the dimension of the chaotic runoff time series of reservoirs.The Genetic Algorithm(GA) and ANN were combined together to repair the shortcoming of each other.The new method has been applied to calculate and forecast the chaotic runoff of the Sanmenxia Reservoir.Contrast to the normal BP networks,the calculation error of the new model is reduced,and the convergence speed is improved.