Abstract: The Muraenesox cinereus and other eels of Anguilliformes are common near-bottom ferocious carnivorous fish in the waters of the Zhoushan Islands. Studying the trophic niches of eels can provide an understanding of the trophic levels they occupy in the marine ecosystem of Zhoushan Islands. The Muraenesox cinereus, Dysomma anguillare, Conger myriaster, Pisodonophis cancrivorus and Rhynchocongerbrevirostris caught from the waters of the Zhoushan Islands were selected as the research objects in this study. The carbon and nitrogen stable isotope technology was used to analyze the trophic niche and interspecies diet relationship of five eels. A total of 143 samples of 5 kinds of eels’ carbon and nitrogen stable isotopes were determined in this study. We found that the total anal length of the five eels varies from 43.00 to 383.00 mm, with an average of 157.14 mm. And the weight varies from 15.60 to 1184.50 g, with an average of 150.07 g. The δ¹³C range of the overall sample is −21.91‰ to −14.97‰, with an average value of −17.03‰±1.14‰, while the δ¹⁵N range is 9.56‰ to 15.13‰, with an average value of 11.50‰±1.36‰. The results of Non-Parametric test showed that there were extremely significant differences in the stable isotope ratios of carbon and nitrogen among different species. Pearson correlation analysis showed that, the δ¹³C and δ¹⁵N values of the 5 species of eels were not significantly correlated with anal length. Except for the D. anguillare, the δ¹³C value of the remaining four types of eels were not significantly correlated with the anal length, and the δ¹⁵N value were extremely significantly positively correlated with the anal length. The overall trophic levels of the five eels range from 3.16 to 4.79, with an average value of 3.73. The SIAR showed that M. cinereus mainly feed on fishes, shrimps and stomatopoda, while D. anguillare mainly feed on shrimps and stomatopoda. Fishes were the main food of C. myriaster, and cephalopods were secondary. P. cancrivorus mainly fed on fishes, shrimps and Cephalopods, while R. brevirostris fed on fishes, shrimps and stomatopoda on average. The nutritional niche of M. cinereus overlaped with the other 4 species of eels. It indicated that the diet sources of M. cinereus and other species have a certain degree of homogeneity, and there was food competition. The corrected version of the Standard Ellipse Area (SEAc) of P. cancrivorus and R. brevirostris were the two largest values, indicating that the eating habits of the two are generalized, and the feeding groups mainly depend on the availability of food organisms. There was no niche overlap between the D. anguillare, C. myriaster, and R. brevirostris. It showed that there are differences in their feeding source, which reflected the differences in the feeding habits of related species in sympatric distribution. This study described the interspecies diet relationship and evolutionary coexistence mechanism of the five species of eels in the waters of the Zhoushan Islands, which can provide a basic reference for the research on the characteristics of the food web structure and the resource effects in the waters of the Zhoushan Islands.