Abstract: Taking Pinghutang River in Jiaxing City and its surrounding river waters as the research area,taking ammonia nitrogen and BOD5content in water as water quality indexes,the coupling model of hydrodynamic and water quality of plain river network is established by using MIKE21,and the migration and diffusion process of pollutants in the river network model is simulated. By analyzing the accident discharge process under different working conditions,it is concluded that under certain conditions,the diffusion area of pollutants per unit time is obviously linear with the diffusion time. When other conditions are the same,the diffusion area of pollutants at the same time has a four-time correlation with the concentration of accident discharge. Under the condition of keeping the total emission constant,the diffusion area of pollutants in a certain period of time has a great relationship with the emission pattern. The results show that the model has good adaptability and high accuracy in this study area. The results can provide reference for simulation analysis of pollutant diffusion in plain river network.