To explore the water diversion control effect in complex hydro-hydrodynamic conditions and polluted source areas, the downstream river network area in Taihu Lake Basin was selected as the research object. Based on the monthly monitoring datas of NH₃-N and COD_Mn in 29 sections of the study area from 2007 to 2018, single-factor evaluation method, comprehensive pollution index method, cluster analysis method, Pearson correlation analysis and one-way ANOVA were used for analysis. The results show that the water quality in the study area can be divided into two periods in terms of time with 2015 being the inflection point for the overall improvement of water quality. Sections can be divided into six categories in terms of space, with the section closer to the water intake having a better improvement effect and the scope of improvement expanding by water diversion year by year. Different water quality indicators have different responses to water diversion quantity with NH₃-N significantly negatively correlated with water diversion quantity in the section closer to the water intake, while in more sections, COD_Mn has negative correlation with water diversion quantity. There are spatial differences in the influence of different water diversion quantities on the water quality index. It is concluded that when the water diversion quantity is 200 million-250 million m³, the improvement effect of water quality is the best.