Gate-controlled operation is a crucial strategy for water resources management in coastal plain river networks. However, the strong hydrodynamic disturbances it triggers can easily induce sediment resuspension, leading to a surge in Sediment Oxygen Demand (SOD), which subsequently alters dissolved oxygen (DO) dynamics and induces hypoxia risks. To reveal the response mechanism between hydrodynamics and SOD under gated conditions, this study takes Jinhuigang River in Shanghai as a case study. Based on in-situ hydrological and water quality monitoring and sediment sampling, a one-dimensional hydrodynamic model considering tributary effects was constructed. Combined with an SOD prediction model established via sediment oxygen consumption kinetic experiments, the coupling characteristics of hydrodynamic conditions and SOD under different water diversion intensities were systematically analyzed. The results indicate that: (1) The constructed one-dimensional model effectively simulates the hydrodynamic evolution process in the gated channel, accurately capturing the longitudinal propagation and lag effects of hydrodynamic disturbances; (2) Gate-controlled water diversion significantly enhances channel hydrodynamic intensity, with SOD showing an overall increasing trend as the diversion scale increases, while exhibiting significant spatial heterogeneity; (3) Under strong hydrodynamic disturbances, SOD shows a stable positive linear response to flow velocity. Flow velocity acts as the dominant mechanism amplifying SOD by increasing the equivalent suspended sediment thickness (hs) and accelerating the exposure and release of oxygen-consuming substances. This study reveals the key response chain of "hydrodynamic enhancement—sediment resuspension—SOD amplification" under gated scenarios, providing a theoretical basis for refined water quality operation and hypoxia risk prevention in plain gated rivers.