Based on the hydrodynamic and water quality characteristics, and pollutant load of the river networks in Taizhou City, the software MIKE-11 was adopted to establish a one-dimensional hydrodynamic and water quality coupling model. The model’s parameters were calibrated and verified in terms of the measured water level, NH₃-N, and COD concentrations in the river networks. On the basis of the model, the water environmental capacity and the pollutant load were calculated. In addition, the pollutant load duration curve for the rivers in Taizhou City was built. The reduction amounts and reduction rates of NH₃-N and COD in all flow duration regions were calculated. This study aims to provide a basis for water environmental protection and water resources management in plain river networks.