Taking the three water circulation systems of Zhangjiagang City as an example, a one-dimensional river network mathematical model of hydrodynamic force and water quality was constructed, which revealed the response relationship between different tidal levels of the Yangtze River and the amount of water diverted from the inland river, and the response relationship between different amounts of water diverted and the effect of water quality improvement and the service area of water diversion. The results show that with the rising of the high tide level of the Yangtze River, the water diversion volume of the inland river presents a linear growth trend. When the water diversion volume of the middle water system reaches 1. 4×10⁷ m³, the improvement rate of ammonia nitrogen concentration is more than 50%, the concentration change index is more than 0. 6, and the overall water quality category is improved by one degree. The service area of water diversion shows a linear growth trend with the increase of water diversion volume, while the service area of unit water diversion volume shows a logarithmic function decline trend. The service area of unit water diversion volume of northeast water system is the largest, which is 3. 52×10^-5~8. 91×10^-5 km²/m³.