Abstract: The key of large-scale system optimization is dimension reducing of models. The mixed-model is a method of dimension reducing by means of decomposing the total system into several sub-systems with loose inter-connections. The principies of the model construction, the classification, the characteristics and the optimal methods of the mixed model are summarized theoreticaly in this paper. It shows thatthe mixed model is useful applicated in the large water resources systems and irrigation or drainage systems with complicated structures and multiple functions.