Abstract: In order to improve the management capacity on emergency of relevant departments during storms in Guangzhou urban area, SWMM (Storm water Management Model) was applied to simulating the rainfall-runoff process. The model parameters were calibrated based on the water elevation monitoring in the storm pipes during an observed storm event. Another observed storm event was used to verify the parameters, demonstrating that the model can estimate the flow in the drainage system with acceptable accuracy. The model was embedded into ArcGIS by Python to establish a early-warning system based on real- time simulation of the rainfall-runoff process. The direct presentation of the simulation results in ArcGIS helps with emergency schemes implementation.