In order to predict the future development trend of Coilia nasus and assess its impact on reservoir ecosystem in the Three Gorges Reservoir, we primarily investigated the trophic level of C. nasus and other typically omnivorous and carnivorous fishes, which were collected from the Caotang River and Daning River. Then, C. nasus, four fish species with similar trophic levels (i.e., Neosalanx taihuensis, Culter alburnus, Elopichthys bambusa, and Siniperca kneri), and one fish species with similar feeding characteristics (i.e., Aristichthys nobilis) were chosen, and their trophic niche characteristics were investigated based on the stable carbon and nitrogen isotope analysis. The results indicated that the range of δ¹⁵N value of C. nasus was 12.112‰-14.497‰, and its trophic level was calculated as 3.762±0.196 (Mean±SD), which was lower than that of N. taihuensis and higher than those of C. alburnus, E. bambusa, S. kneri, and A. nobilis. The range of δ¹³C value of C. nasus was from -27.236‰ to -22.952‰, and its variation range of δ¹³C value was less than that of A. nobilis and larger than those of N. taihuensis, C. alburnus, E. bambusa, and S. kneri. The distribution frequency of δ¹³C value of C. nasus was unimodal, which was similar to that of N. taihuensis. These results clearly indicated that C. nasus could consume a wide range of food sources in the Three Gorges Reservoir, and its feeding preference was similar to N. taihuensis. Therefore, it was suggested that the higher trophic level of C. nasus might be due to the extensive food sources and more abundant enrichment pathway of δ¹⁵N in the Three Gorges Reservoir. The results of trophic niche analysis showed that the trophic niche breadth of C. nasus was narrower than those of E. bambusa and A. nobilis, and wider than those of N. taihuensis, C. alburnus, and S. kneri. The results indicated that the C. nasus might not have a high degree of specialization in the bait demand, and had strong adaptability to the reservoir ecosystem of the Three Gorges Reservoir. Besides, the trophic niche overlap between C. nasus and other fish species with similar trophic levels was low, and there was an obvious niche differentiation. Overall, our results suggest that there is a possibility of further expansion of C. nasus population in the Three Gorges Reservoir, and these results will provide basis for understanding the adaptation strategies and exploring the risk assessments of exotic fish species in the Three Gorges Reservoir region.