Abstract: The water environment of plain river networks has received widespread attention, and ecological water diversion schemes are often used to improve water quality. For such complex river networks, many hydrodynamic–water quality simulations have been conducted; however, a quantitative evaluation method for the effects of water diversion projects is urgently needed. Taking the central urban area of Haining as a case study, a one-dimensional hydrodynamic–water quality numerical model was developed, and nine water diversion schemes involving pumps and sluices were simulated. An integrated evaluation framework combining the AHP, CRITIC, and revised TOPSIS methods was used to quantitatively assess the effects of water diversion schemes. The results indicated that Scheme 6 （in which water was diverted through the Changshanhe East Pump and Xiaoshihe Pump, with the middle sluice closed and drainage through the Majinggang South Sluice and Yuanxiahe Sluice） achieved the best overall performance, with compliance rates of ecological flow and NH₃-N of 48.1% and 94.9%, respectively. Compared with existing methods, the improved TOPSIS method effectively reduced the redundancy caused by interior correlations among indicators and better identified the optimal scheme. The relative closeness of Scheme 6 （0.732） was the highest among those of all the schemes, consistent with the simulation results. Overall, the proposed evaluation approach could be used to improve the scientific validity and reliability of water diversion scheme optimization and provide technical support for water environment management and regulation in plain river networks.