Abstract: To reveal the influence mechanism of the layout of effective impervious area (EIA) on urban stormwater processes, a refined stormwater model was constructed. Based on the pipe network concentration time, sub-catchments were divided into upstream and downstream zones. Taking the Shuangqiaobang runoff plot in Changzhou, southern Jiangsu, China, as a typical study area, the impact on peak flow and runoff volume were systematically analyzed by setting different scenarios of EIA increments with different locations and scales. Results showed that the location of the EIA, infiltration capacity of the original pervious surface, and rainfall magnitude are key factors affecting the hydrological response. The spatial location effect of newly added EIA is significant, with the growth rates of peak flow and runoff volume in downstream areas being significantly higher than those in upstream areas, showing maximum difference of 18.48% and 7.70%, respectively. Pervious areas with stronger original infiltration capacity exhibit a greater proportional increase in runoff after conversion to EIA. The growth rates of peak flow and runoff volume decrease with increasing rainfall return period, exhibiting an amplification effect under low-return-period rainfall events. This study provides a scientific basis for optimizing the spatial patterns of underlying surfaces in urban settings and guiding the rational layout of stormwater facilities.