The hydrodynamic of river channels in plain cities are generally of poor conditions. Accelerating water renewal and strengthening orderly flow is one of the most important ways to improve the water environment quality of river network. In this paper, a typical plain city in the lower reaches of the Yangtze River, Qidong, was studied as an example. Making full use of the natural tidal dynamic conditions, a hydrodynamic model of the river network was established. Numerical simulation experiment was carried out to quantify the improvement of hydrodynamic characteristics of the river network through water system pattern and overall arrangement optimization and water conservancy project operation. The results show that, the optimized scheme increases the ecological velocity compliance rate of the river network from 52.3% to 94.2% for the whole period under the worst tidal condition; the maximum instantaneous ecological velocity compliance rate is raised from 42.6% to 85.0%;the regional ecological velocity continuity increases to 91.8%. Meanwhile, the urban ecological water consumption reduces 67.5%. Under the optimized scheme, the diversion water volume from external regions can be reduced, and the renewal rate of urban river network significantly increases which shrinks the accumulation time of pollutants. This study provides a novel perspective and method for improving the water environment of tidal river channels in plain cities.