Abstract: The stable isotope ratios (δ¹³C and δ¹⁵N) of carbon and nitrogen of Tachypleus tridentatus collected from Beibu Gulf and its potential food source species (Shrimps, crabs, bivalves and fishes) in food web were determined. Besides, their food composition and nutritional position were anayzed by stable isoelement mixture model (SIAR). The results show that the average δ¹³C and δ¹⁵N values of the T. tridentatus were (−17.11±0.03)‰ and (12.14±0.20)‰, respectively. Potential food sources of T. tridentatus were shrimps, crabs, bivalves and fishes. The SIAR model shows that bivalves were the main food source of T. tridentatus with average contribution rate of 31.27% (Fishes: 25.91%, crabs: 23.50%, shrimps: 19.32%). The trophic position of T. tridentatus was 2.02±0.06, while those of fishes, shrimps and crabs were 2.13±0.51, 2.36±0.09 and 2.61±0.21, respectively. Trophic position analysis shows that T. tridentatus is in a relatively weak position in the natural ecosystem food web, and scientific protection measures are urgently needed as it is already endangered.