To explore the feeding habits and trophic niche of mesopelagic fish in the adjacent sea area of Xisha in the South China Sea, this study focused on four mesopelagic fish: Diaphus splendidus, Diaphus mollis, Diaphus fulgens, and Diaphus luetkeni. Carbon and nitrogen stable isotope technique was used to investigate the trophic levels, trophic niches, and primary food sources of these four species. The results showed that: (1) The δ¹⁵N values of the four species ranged from 8.31‰ to 11.08‰, and the δ¹³C values ranged from –21.88‰ to –18.19‰; (2) There were differences in trophic levels among the four species, with D. luetkeni (3.75) ＞ D. splendidus (3.63) ＞ D. fulgens (3.57) ＞ D. mollis (3.41); (3) D. fulgens had a largest trophic niche width and a stronger ability to utilize bait resources. The trophic niche width of D. splendidus was the smallest, and which had strong selectivity and feeding specificity towards bait organisms; (4) There was a certain competitive relationship in food resource intake among the four species, but there was no competition between D. luetkeni and D. mollis; (5) Bayesian mixture models indicated that zooplankton was the main food source for the four mesopelagic fish. This study revealed the main food sources and trophic niche characteristics of the four mesopelagic fish species in the adjacent sea area of Xisha, providing basic data for the study of the feeding characteristics and nutritional structure relationships of mesopelagic fish, and was significant for further research on the energy flow and material cycling in marine ecosystems.