In response to the potential spatial transfer of flood risks caused by the improvement of urban drainage capacity and the adverse impact on flood control safety of external drainage channels, a refined hydrological and hydrodynamic model was constructed in the central urban area of Wuxi City and adjacent Jiangnan Canal as the study area. The scenario analysis method was used to compare the differential effects of three drainage modes, namely natural gravity drainage, current pump station drainage, and increased pump station capacity drainage, on flood risks inside and outside the city. The results show that the use of pumping stations can significantly reduce the peak water level in urban areas and shorten the duration of exceeding the warning water level. However, under the existing scheduling rules, the water level improvement effect brought by further improving the pumping station capacity is no longer significant. The momentum exchange effect generated by the concentrated strong discharge at the mouth of the discharge channel in Wuxi City forms a hydraulic barrier along the way, leading to flow reversal and water level rise in the upstream section, while the downstream section is less affected. When the base flow rate of the discharge channel is low, forced lateral inflow is more likely to change the original flow state of the channel, showing obvious low flow sensitivity characteristics.