Flood routing system is considered as a single input-output system in this paper.Stage routing model and stage forecasting method,based on ARMAV(2,1) model with time series analysis,is derived using the optimal control theory.The following conclusions can be given from this study:if stage is variable,stage routing parameters are then identified using optimal method without restraint condition;if water flow depth is variable ,the parameters are determined by the use of the optimal method under the water balance condition.The method has been applied to the stage prediction on the lower reaches of the Yellow River,the good results are obtained in both model calibration and verification.