Urban shrinkage， as a phenomenon accompanying urbanization， has begun to emerge in mountainous cities in central and western China. Objectively and accurately identifying the characteristics of urban shrinkage and exploring its underlying mechanisms are crucial for coordinated regional development. This study takes 38 county-level administrative units under the jurisdiction of Chongqing Municipality in 2023 as research objects and constructs a three-dimensional framework integrating population， economy， and space. By combining spatial autocorrelation analysis， k -means clustering， and geographically weighted regression， it investigates the spatiotemporal differentiation， spatial agglomeration patterns， and influencing mechanisms of urban shrinkage across Chongqing’s counties from 2003 to 2023. The findings reveal: 1) The spatiotemporal pattern of urban shrinkage in Chongqing exhibits a polarized structure characterized by “core growth—peripheral shrinkage” with growing districts highly concentrated in the main urban area， while shrinking districts are mainly located in the Three Gorges Reservoir region of northeastern Chongqing and the Wuling Mountain area of southeastern Chongqing. 2) There is significant spatial association in urban shrinkage， with high-high (hotspot) clusters extending from the core of the main urban area toward adjacent suburban counties， while low-low (coldspot) clusters expand continuously in the Three Gorges Reservoir region of northeastern Chongqing. 3) k -means clustering identifies four distinct types of shrinkage during the entire study period， consistent with a gradient structure of “core—transition—periphery”. 4) Factors influencing urban shrinkage in Chongqing’s counties exhibit marked spatial heterogeneity， with income levels and industrial development being key driving factors that suppress shrinkage， while elevation serves as the primary geographical constraint factor promoting urban shrinkage. Based on the above findings， this study proposes differentiated response strategie, including precise classification governance， strengthening endogenous driving forces overcoming mountainous constraints， and improving collaborative mechanisms, with the aim to provide theoretical and practical references for formulating differentiated development strategies in Chongqing’s counties and similar studies in China.