Abstract: This paper develops an optimal water supply operation model of pluriennial regulation reservoir groups with external water resources, which has been applied in a case study of a reservoir groups water diversion system for one city. By integrating water diversion and water supply into an optimization system, this model reduces the system scale and dimensionality and implements global optimization with a dynamic programming method. We describe a concept of year-end water storage target of pluriennial regulation reservoir and an optimization method for determination of its value, so as to combine the factors of annual operation and pluriennial regulation. Relative to conventional operation, the optimized operation reduces water transfer by a sum of 8.44×106m3 and total waste water by 2.99×106m3. Thus, the storage of reservoir groups is increased substantially at the end of the year, and water supply capacity in low water season also increased, effectively reducing the degree and extent of water supply interruption. Comparison shows that the model and optimization method can improve water supply security in low water season while fully exploiting the potential of water resources and reducing the cost of water supply.