Abstract: This study develops a high-resolution numerical model by bidirectionally coupling of 2D overland flow with 1D pipe flow to clarify the build-up, wash-off and transport mechanisms of urban non-point source pollutants. This model utilizes a power law build-up equation and wash-off module was seamlessly integrated into a proprietary 2D hydrodynamic engine. The coupling framework, executed through a dynamic link library (DLL) interface, ensures real-time dynamic exchanges between the surface and pipe systems, while GPU acceleration overcomes the computational bottleneck inherent in high-resolution 2D simulations. This model was validated using a combination of roof-scale physical experiments and multi-event field monitoring data from a representative campus catchment. The findings indicate that the model effectively simulates the build-up, wash-off, and transport processes of total suspended solids (TSS) at a small catchment size with elevated spatial and temporal resolution. The Nash-Sutcliffe efficiency coefficients for the simulated TSS concentrations in roof runoff, as well as the water levels and TSS concentrations at pipe nodes, are all above 0.7. It provides a comprehensive, mechanistic characterization of the full TSS lifecycle, including generation, routing, transport, and discharge. Thereby offering robust technical support for investigating urban non-point source pollution mechanisms and implementing targeted mitigation strategies.