Taking the Wunnan Area of Shanghai City, a typical plain river network, as an example, this study comprehensively adopted technical methods such as measured data analysis, numerical simulation, and on-site sampling and determination to conduct research on the mechanism of water quality deterioration in tidal-influenced river networks. The results show that:the water quality of the river network in the Wunnan Area exhibits significant spatiotemporal heterogeneity-water quality in the non-flood season is significantly better than that in the flood season, and water quality in the eastern and southern rivers is significantly superior to that in the western and northern rivers；rainfall causes severe exceedances of indicators such as NH₃-N、TP、COD, and turbidity in the regional river network, among which the increased pollution load from direct sewage discharge induced by rainfall is the primary cause of water quality deterioration in the flood season；direct sewage discharge also leads to serious sediment pollution in the regional river network；during the flood season, engineering operations aimed at flood control form a hydrodynamic pattern of low water level-high flow velocity, and sediment scouring, pollutant resuspension, and continuous release further exacerbate water quality deterioration in the flood season; in the non-flood season, low-dynamic rivers, due to low water exchange rates and weak self-purification capacity, coupled with the continuous release of pollutants from sediment, result in water quality deterioration and poor stability.