[Objective] This study assesses the waterlogging risk of the bus system in Tianjin’s central urban area using a zonal layered numerical simulation model. The systematic assessment includes identifying risks at bus routes and stops，analyzing the spatial distribution of waterlogging risk，and quantifying the length of inundated road sections. [Methods] The waterlogging disaster risks on bus routes in Tianjin’s central urban area were assessed and analyzed under short-duration （3-hour） and long duration （24-hour） design storm hyetographs. The Pilgrim & Cordery hyetograph was used for the short-duration design storm，and a frequency-consistent hyetograph design method was used for the long-duration one. A zonal layered mathematical model for urban rainstorm waterlogging was established，incorporating both two-dimensional planar and three-dimensional spatial simulation approaches to account for the heterogeneity of the urban underlying surface. This model coupled hydrodynamic modules for communities，roads，rivers，and pipe networks，enabling refined simulation of the central urban area and the prediction of waterlogging risks on road sections. Using design storms of various return periods （for both 3-hour and 24-hour durations） as precipitation boundaries，and based on the current status of drainage facilities，the model simulated the waterlogging risks on bus routes within the area inside the Outer Ring Road under different return periods. [Results] 1） Case study simulations of waterlogging risks on bus routes were performed for two rainstorm events of different intensity. In terms of the simulation of key road sections，12 out of the 14 waterlogged and closed roads announced by the traffic management department during Event 1 were simulated to meet the traffic interruption criteria （i.e.，waterlogging risk at Level 3 or above）. During Event 2，all four simulated underpass interruption points met the criteria. The model demonstrated high accuracy in simulating road traffic interruption risks and could well match the actual waterlogging-induced traffic interruption situations. From the perspective of simulating the waterlogging risk distribution of individual bus routes，the model could clearly reflect the spatial distribution characteristics of waterlogging risks on a single bus route，providing customized support for the refined assessment and management of waterlogging risks on individual bus routes. 2） Under the scenarios of design storms with different return periods，the impacts of short-duration （3-hour） and long-duration （24-hour） rainfall on waterlogging risks of bus routes were simulated，and the waterlogging risks of bus routes under various return periods were further analyzed. The results indicated that the number of bus routes and stops with Level 1 waterlogging risk caused by short-duration rainstorms was larger than that caused by long-duration ones. In contrast，the number of bus routes and stops with waterlogging risk at Level 2 or above induced by long-duration rainfall was higher than that induced by short-duration rainfall. Under each return period，the maximum rainfall intensity of the 24-hour duration was greater than that of the 3-hour duration，which resulted in more prominent waterlogging risks at Level 2 or above under long-duration rainfall. Meanwhile，the relatively long duration of short-duration intense precipitation led to a wider impact range of Level 1 waterlogging risk. [Conclusion] Bus routes with high waterlogging risks in the central urban area are highly agglomerated in six old urban districts，where the public transit network is densely distributed. Once a waterlogging disaster occurs，it is prone to triggering severe public transportation paralysis. The simulation results from two rainstorm case studies validated the model’s effectiveness. It accurately identified key points of transit service interruption and mapped the risk distribution along individual routes，thereby providing a scientific research method for the flood control and dispatch of public transit systems. Restricted by the current status of built-up areas，it is recommended that the drainage capacity of waterlogging-prone road sections should be improved by constructing new pumping stations and adding temporary drainage measures. Meanwhile，traffic management department should establish a mechanism for waterlogging risk assessment and emergency response of public transit routes based on the early warning information from multiple departments，formulate transit flood response plans，and timely adjust or suspend the operation of transit routes to avoid losses.