Abstract: Ceratoscopelus warmingii and Bolinichthys longipes are common mesopelagic fish in cold seep ecosystems, serving as crucial links between surface and deep-sea ecosystems. To explore the food web and material cycling flow in cold spring ecosystems, we collected samples of these mesopelagic fish from the adjacent waters of the cold seep area in the South China Sea in August 2021, and then analyzed their trophic niche characteristics using stable isotope techniques. Results indicate that the δ¹³C, δ¹⁵N and the trophic level of C. warmingii in the cold seep area were (−20.22±0.35)‰, (8.6±0.48)‰ and 3.35±0.14, respectively, while those in the control area were (−20.51±0.33)‰, (8.15±0.65)‰ and 3.40±0.19, respectively. Similarly, the δ¹³C, δ¹⁵N and the trophic level of B. longipes in the cold seep area were (−20.19±0.28)‰, (9.32±0.66)‰ and 3.57±0.19, respectively, while those in the control area were (−20.68±0.42)‰, (8.86±0.68)‰ and 3.61±0.20, respectively. The δ¹³C and δ¹⁵N values of B. longipes were higher than those of C. warmingii in both regions, and the proportion of B. longipes feeding to secondary consumers such as fish and shrimp was higher. It is showed that the food source was wide, the trophic level was high, and the competition and utilization ability of resources was stronger. Both species showed lower trophic diversity (CD) and narrower trophic niche (SEAc) in the cold seep area, because consumers could be selective in feeding due to the unique ecological environment and high biodiversity of cold seep. Meanwhile, cyclic statistical analysis shows that the trophic structure of both species of mesopelagic fish changed towards the cold seep area, indicating that the trophic structure of the mesopelagic fish population in the cold seep area was more complex and stable.