Abstract: The Chinese Merganser (Mergus squamatus) is listed as a Class I National Key Protected Wildlife Species in China and classified as Endangered on the IUCN Red List. Effective conservation of overwintering habitats is essential for maintaining its population size and achieving species persistence and population recovery. Anji County, situated in the lower Yangtze River basin of Zhejiang Province, possesses a well-developed water network and favorable vegetation conditions, making it a vital overwintering site for this species. Currently, local research on overwintering habitat suitability for the Chinese Merganser remains scarce, limiting the ability to provide solid support for the formulation of refined conservation strategies. This study takes the Chinese Merganser in Anji County as the research subject, predicts its potential suitable overwintering distribution, and identifies dominant environmental driving factors, aiming to offer a scientific reference for the conservation and management of this endangered species. Based on 82 valid occurrence points obtained from field monitoring between 2019 and 2024, 12 environmental variables spanning four categories of climate, topography, land use, and human disturbance were selected, and the MaxEnt model was applied to evaluate habitat suitability. Factor contribution rates and jackknife tests were jointly used to screen the core influencing factors. 10 cross-validation produced a mean AUC value of 0.988 for the model, indicating strong predictive performance and reliable overall simulations. The results showed that the main environmental factors constraining the overwintering distribution of the Chinese Merganser and their relative contributions were river network distribution (47.0%), land use type (28.5%), precipitation seasonality (14.7%), and elevation (4.9%). The species′ ecological response patterns revealed that it predominantly inhabits shallow waters at elevations of 75—125 m with stable precipitation and weak hydrological variability. Water bodies and mudflats serve as its core habitat carriers, while human-dominated land use types such as construction land and cropland substantially compress its living space and reduce habitat occupancy probability. The predicted high-suitability areas were not uniformly distributed across Anji County but were highly concentrated in the tail-end mudflats, sinuous shorelines, and inflow zones of the three major reservoirs: Fushi, Laoshikan, and Fenghuang. These areas feature slow-flowing shallow water, abundant prey resources, and weak human disturbance, constituting the core overwintering habitats for the species. This study clarifies the dominant controlling factors and spatial distribution characteristics of the Chinese Merganser′s overwintering habitat in Anji County, accurately delineates the extent of high-suitability habitats, and provides a theoretical basis for local in-situ conservation of this endangered species. Future studies should incorporate micro-habitat indicators such as water quality and fish abundance to refine model parameters and further enhance the model′s ecological interpretability.