To characterize the trophic structure of the Chenhang Reservoir ecosystem in the Yangtze River Estuary， this study utilized carbon and nitrogen stable isotope analysis to examine primary carbon sources， trophic levels， trophic structure indices， and niche characteristics of fish communities in 2023. The results demonstrated that the fish δ¹³C values in Chenhang Reservoir ranged from -27.19‰ to -20.46‰， with a mean value of -25.27‰±1.91‰， showing significant differences （P<0.05） from organic detritus （-14.53‰±0.50‰） but aligning closely with phytoplankton， zooplankton， and aquatic plants.Notably，Xenocypris davidi exhibited distinctly higher δ13C values （-20.46‰±0.59‰） compared to other species. The trophic levels of fish with different feeding habits were as follows： secondary carnivorous fish （3.8±0.09）>primary carnivorous fish （3.17±0.19）>herbivorous fish （2.84±0.25）>Planktivorous fish （2.57±0.10）>omnivorous fish （2.53±0.25）. Notably， herbivorous species including Megalobrama amblycephala（3.02） and Parabramis pekinensis （2.74） displayed higher trophic levels than filter-feeding Hypophthalmichthys molitrix （2.64） and Aristichthys nobilis （2.5）.The fish community exhibited intermediate trophic range （NR=5.21）， carbon source diversity （CR=7.24）， core niche space （SEAc=12.77）， and average trophic diversity （CD=1.54）. However， species aggregation density （MNND=1.21） and evenness （SDNND=0.78） were relatively high， while trophic redundancy remained low. These findings indicated that plankton served as the predominant carbon source for most fish consumers in Chenhang Reservoir， although X. davidi additionally incorporated substantial organic detritus. The elevated trophic positions of herbivorous fish likely reflected limited aquatic plant availability， resulting in a dietary transition toward more omnivorous feeding strategies. The findings of this study may serve as a scientific foundation for reservoir water quality management and ecological regulation strategies.