Abstract: Jellyfish (Rhopilema esculentum), as a large zooplankton, plays an essential role in the energy flow of marine ecosystems. In this study, we applied the fatty acid biomarker method and carbon and nitrogen stable isotope technique to investigate the diet and trophic level of cultured jellyfish in Tongzhou Bay. The results show that there were 29 kinds of fatty acids in jellyfish, including 10 kinds of saturated fatty acids (SFA), 8 kinds of monounsaturated fatty acids (MUFA) and 11 kinds of polyunsaturated fatty acids (PUFA). The specific fatty acid analysis reveals that diatoms, terrestrial plants, herbivorous copepods and benthos dominated in the diet of cultured jellyfish in Tongzhou Bay, followed by planktonic bacteria. Besides, the DHA/EPA value was 0.78<1, which indicates that jellyfish had a lower trophic level. The δ¹³C value ranged from −23.54‰ to −20.75‰, with an average value of (−22.26±0.66)‰.The range of δ¹⁵N was 8.39‰−9.85‰ with an average value of (9.02±0.29)‰. No significant correlation is detected between the diameter of jellyfish and δ¹³C and δ¹⁵N (P>0.05), showing that there is no significant change in the trophic level along with the growth of adult jellyfish.