In view of the prominent contradiction between the accuracy and calculation efficiency of the numerical simulation of rainstorm floods in high-density cities, based on the characteristics of short-term heavy rainfall, the characteristics of rapid runoff generation and concentration, the limitations and actual needs of refined simulation and other current situations, starting from the whole basin system, according to the principle of water balance and the equivalent flow capacity of the pipeline, the dimension reduction generalization of the pipeline network was carried out, and the calculation method of the equivalent pipeline parameters was given. Taking the Liedechong River Basin, Guangzhou City, as an example and the applicability of the model was evaluated by using the measured waterlogging submergence depth, location, and the process water level of the manholes and river channels. The results show that the Nash efficiency coefficients of the coupled model based on pipeline network dimensionality reduction generalization in simulating the dynamic changes of water levels in river channels and manhole of the Liedechong River Basin are all greater than 0.7. The proposed pipe network dimensionality reduction generalization method is reasonable. The relative error of surface inundation depth is 8.93%, and the absolute error is 0.05 m. The simulation speed has increased by 57% compared to the coupling model based on the current situation of the pipeline network. The simulation results of the coupled model based on pipeline network dimensionality reduction generalization are in good agreement with the actual situation, and can better reflect the evolution process of urban floods, with hiqh accuracy and efficiency.