In recent decades, stable isotope techniques have been increasingly applied in studies of the diet and trophic structure of birds. Understanding avian diets is critical for the rehabilitation and conservation of endangered bird species, especially for waterbirds like Saunders's Gull. The Shuangtaihekou National Nature Reserve accommodates the largest breeding population of Saunders's Gulls in the world, and the gulls breed in the reserve's man-made wetlands. The objective of this study was to determine the diet composition of Saunders's Gull, using stable isotope analysis to ensure the best management practices of this globally vulnerable species. A total of 213 samples of six potential dietary species, 10 fledgling blood samples, and 27 and 17 feather samples from fledglings and adults, respectively, were collected. The results showed that: (1) The stable carbon isotope signature (δ¹³C) of feathers from adult and fledgling gulls were significantly different. The feathers of fledglings provided short-term feeding information at the breeding site, whereas the feathers from adult gulls provided long-term information that also included feeding at stopover sites. (2) The δ¹³C and δ¹⁵N values of feathers from diseased and dead birds were no different than those from healthy birds, and there was also no difference between the isotope values of the blood and feathers of fledglings. Therefore, in future studies, we can conduct stable isotope analysis using feathers from deceased birds, rather than taking blood and feather samples form healthy birds. (3) In addition, the diet composition of fledglings revealed by blood samples was consistent with that revealed by feather samples. Bullacta exarata and Nereis succinea were the main food sources, and each accounted for over 31.20% of the gull's diet. The next most important contributors were Synechogobius hasta (12.86%-14.49%), Macrophthalmus dilatatum (8.08%-9.19%), and Helice tientsinensis (6.00%-7.48%). With continuous loss and degradation of natural wetlands, the results also provide a scientific basis for the construction, restoration, and management of dietary species for breeding populations of Saunders's Gulls in man-made wetlands.