[Objective] The Storm Water Management Model （SWMM） is widely used in urban flood simulation and prediction. Existing studies have primarily focused on drainage systems in plain urban areas，with relatively limited attention to the composite hydrological systems of coastal piedmont cities. This study aims to identify the core driving factors of model responses under different land use types and quantitatively analyze the variation characteristics of dominant parameters with increasing rainfall intensity，providing a scientific basis for constructing high-precision urban flood simulation models and parameter calibration. [Methods] Taking the Jiangbei District of Fuzhou as a case study，we investigated the dynamic evolution of parameter sensitivity within mountainous （Bayi Reservoir） and urban （Qinting Lake） catchments across varying return periods （5-50 a） using the SWMM model. [Results] （1） The modified Morris method revealed a sensitivity transition in the dominant parameters for peak flow in the mountainous catchment with increasing rainfall intensity. The Horton decay constant dominated under low rainfall intensity （5 a，SN=1.48），while the Manning’s roughness of pervious surfaces （N-Perv） dominated under high rainfall intensity （50 a，SN=0.88）. Conversely，urban areas exhibit a consistent land-surface control effect，primarily governed by the Manning’s roughness for impervious areas （N-Imperv） and depression storage. （2） Sobol global sensitivity analysis indicates that under extreme rainfall，the first-order and total sensitivity indices of mountainous parameters are highly convergent （difference<0.05），manifesting strong parameter independence. In urban areas，however，the total sensitivity significantly exceeds the first-order sensitivity （by 1.5 to 2.5 times），revealing intense nonlinear coupling and interactions between surface runoff and hydraulic processes in the drainage network driven by heavy rainfall. 3） The total runoff in mountainous regions shows extreme sensitivity to N-Perv reflecting the physical mechanism where surface resistance regulates runoff travel time and cumulative infiltration. In contrast，due to the constraints of highly impervious surfaces，the urban runoff response is characterized by multi-parameter joint driving. [Conclusions] Rainfall intensity significantly regulates the sensitivity structure of model parameters，and the response intensity of different physical parameters to model output exhibits a nonlinear evolutionary trend with changes in their own values. Extreme rainfall scenarios effectively amplify the dominant role of core driving factors，revealing the mechanism logic of system transition from runoff generation-dominated to runoff concentration-constrained. This finding provides a physical basis for refined calibration of mountain-urban composite hydrological models and spatiotemporally differentiated parameter validation，contributing to improved flood warning accuracy and the scientific design of waterlogging mitigation strategies in mountainous cities under multi-intensity rainfall conditions.