Abstract: With the rapid development of urban stormwater management informationization, stormwater management models have been widely used in its planning and design. However, the model accuracy has become the bottleneck of the popularization of the modes. In this paper, taking a LID rain water system as the example, the influence of different proportion of the LID system, and different flow paths in low impact development stormwater system on runoff with differerent represent periods, was systematically analyzed using stormwater management model (SWMM). The simulation results showed that the reduction rate of the total amount of rainwater runoff discharge was 4.32%~26.53%, and the peak reduction rate was 38.46%~61.40%, when the confluence path was considered in the simulation. Therefore, the confluence path was proved to be with great influence on the total discharge and peak discharge of rainwater runoff, and it increased with the increase of LID facilities and decreased with the rise of rainfall return period. In the process of site development, we could improve the control effect of LID facilities on stormwater runoff through reasonable confluence path design. And when using the model to evaluate the effect of rainwater control and utilization, the influence of confluence path on the simulation results should also be fully considered.