Abstract: To address issues in urban stormwater simulation due to the lack of detailed drainage network data, such as insufficient modeling of physical processes, difficulty in generalizing drainage capacity, and challenges in simulating sewer overflow and backflow, a method is proposed for approximate simulation of urban stormwater processes in areas lacking detailed drainage network data. This method incorporates a generalized approach to road/storm drain drainage combined with major trunk networks. Based on the GAST-SWMM Model, the method reduces water volume in the road/storm drain areas of a 2-D surface model and routes it to nearby stormwater nodes of the main trunk network for drainage network calculation, achieving approximate simulation of stormwater processes. The method was applied to an urban area in Tangshan City, Hebei Province, and its effectiveness was compared with that of the detailed drainage network coupling model and the main trunk network coupling model. Results indicate that under different rainfall recurrence intervals, the peak surface water error of the two approximate simulation methods compared to the detailed network coupling model is less than 20%, and the peak drainage error is less than 25%, which is an improvement of 10.2% to 32.3% and 53.1% to 69.8% respectively over the main trunk network coupling model. This demonstrates that the proposed method can approximately represent the drainage effects of detailed networks and provide a reference for flood process calculation in urban areas and regions lacking detailed drainage network data.