Abstract: This paper develops a novel intra-urban spatial structure model that extends traditional linear or circular city frameworks to accommodate cities of arbitrary shapes.For the first time,it examines how the number and location of urban sub-centers—key components of a polycentric urban layout—affect residents’ welfare by reshaping the spatial structure within cities.The theoretical model assumes that the number and locations of sub-centers are exogenous variables determined by urban planning as well as historical and geographical factors.Given the locations of these sub-centers,the spatial structure of the city is determined.This structure,in turn,influences commuting costs and agglomeration effects,leading to a redistribution of population and economic activities and,consequently,affecting residents’ welfare. The model reveals that increasing the number of sub-centers reduces commuting costs within the city.However,the dispersion of firms caused by a polycentric layout may weaken agglomeration economies.Urban planners must weigh these trade-offs and consider initial conditions such as natural geography and industrial bases to avoid coordination failures in sub-center development.Taking Chengdu as a case study,the paper calibrates the model and conducts counterfactual analyses to explore how different spatial planning strategies might reshape the distribution of population and economic activities,ultimately influencing residents’ welfare. The counterfactual analyses reveal that in monocentric cities,welfare levels are highly sensitive to changes in congestion factors,whereas polycentric cities are more resilient.Thus,under severe congestion,a polycentric layout offers significant potential to alleviate traffic-related challenges often faced by large cities.Moreover,Chengdu’s latest Land Spatial Master Plan (2020-2035),which integrates the existing western sub-center with the CBD area while prioritizing the development of eastern and southern sub-centers,appears well-positioned to enhance agglomeration effects,improve commuting efficiency,and elevate overall welfare. The findings provide valuable insights for urban policymakers in designing spatial plans.Furthermore,the proposed model and numerical simulation approach can be applied to other cities,offering a robust framework for evaluating and optimizing polycentric urban layouts—an increasingly critical policy instrument in the ongoing process of urbanization.