Abstract: The frequent occurrence of extreme rainfall events in recent years has increasingly become a significant issue affecting the economic and social development of China． In this paper we start from the perspective of actual urban drainage processes and propose an efficient simulation method for urban flood inundation based on the concept of catchment units． This method utilizes grate inlets as outlets to delineate catchment units, thereby calculating urban surface runoff, flood inundation depth, and distribution． The concept of equivalent width is introduced to unify the calculation methods for flow rate and water level in open channels and pressurized pipe flow conditions． Based on the principle of water balance, grate inlets are used to couple urban surfaces with underground pipelines, and orifices are used to couple pipes with rivers, thereby constructing an integrated solving matrix for surface-pipe-river systems． This proposed simulation method was incorporated into the mature Taihu Basin Model and applied to monitoring data from the urban runoff and flood inundation experimental site in Changzhou to evaluate the effectiveness of this method and the reliability of this model, aiming to provide new insights for urban flood inundation simulation．