Abstract: Deep-water reservoirs possess unique hydrological structures, leading to significant differences in food web construction mechanisms and energy flow patterns compared to natural lakes. To clarify the trophic structure characteristics of such systems, this study takes Zhelin Reservoir as the research area. Based on fish survey data from April and July 2021, carbon and nitrogen stable isotope techniques were employed to systematically analyze the trophic levels of fish and the characteristics and overlap patterns of trophic niches among different functional groups. The results show that in Zhelin Reservoir, the δ¹³C values from –33.51‰ to –17.90‰ (–25.52±0.20‰), and the δ¹⁵N values from 5.96‰ to 16.97‰ (13.00±0.20‰). The main fish trophic levels from 1.03 to 4.27 (3.09±0.06). The omnivorous group occupies the core niche through broad-spectrum feeding (CR=15.61‰), but its high overlap rate with carnivorous groups (88%) intensifies community competition. The herbivorous group is resource-limited due to a single carbon source (CR=8.21‰). The study reveals that the food chain length in Zhelin Reservoir is relatively short (top predator: 3.72), a characteristic closely related to seasonal variations in carbon sources and long-term anthropogenic fishing pressure. From the perspective of trophic niches, the findings elucidate the food web structure characteristics of deep-water reservoirs, where fish utilize diverse carbon sources but exhibit limited food chain lengths. This provides a scientific basis for understanding the energy flow mechanisms in artificial reservoirs and optimizing fishery resource management strategies.