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Gangnan Reservoir is an important water source of Shijiazhuang, the seat of northern China's Hebei Province. The irregular shape of the reservoir makes it easy for water to stop flowing in bays, forming dead water areas. Serious water quality problems will ensue. To disclose the influence of a water diversion project on the water exchange in Gangnan Reservoir, this paper sets up a hydrodynamic and water exchange model for the reservoir based on the convection–diffusion theory. According to different diversion flows and wind fields, a total of 12 working conditions was configured to numerically simulate the water exchange in the reservoir. The results show that: the diversion flow is the main factor affecting the water exchange rate. In the main reservoir areas, the water exchange rate increases gradually, finally, it can reach a relatively high level. In the bay areas, the water exchange rate is relatively small, and affected by the diversion flow and the bay location. The wind field has different impacts on the water exchange rate in the main reservoir areas and the bay areas. The impact is small in the former areas, yet significant in the latter areas. When it is not windy, the residence time decreases with the increase of the flow. When it is windy, the wind field of each direction shortened the residence time in the bay areas, while increased that in the main reservoir areas. Furthermore, the transport and diffusion law of water pollutants in Gangnan Reservoir was analyzed by the connectivity matrix: When the main reservoir areas are polluted, the pollutants greatly affect the main reservoir areas; When the bay areas are polluted, the pollutants only affect some adjacent water bodies. The research findings provide scientific reference for the diversion operation of Gangnan Reservoir. [ABSTRACT FROM AUTHOR]