Abstract: To recognize the fish composition of cold seep ecosystems, and provide a scientific basis for biodiversity conservation and monitoring in the relevant regions, we investigated the mesopelagic fish resources inside and outside the cold seep in the Xisha Islands by a mesopelagic trawler. A total of 106 species of mesopelagic fish had been identified which belong to 62 Genera, 31 Families and 12 Orders, with the dominant groups of Myctophiformes and Stomiiformes. Among them, 84 species of fish belonging to 54 Genera, 30 Families, 11 Orders were collected inside the cold seep; while 60 species of fish belonging to 32 Genera, 13 Families and 6 Orders were collected outside the cold seep. There were 38 shared species of mesopelagic fish belonging to 22 Genera, 9 Families, 5 Orders inside and outside the cold seep, and the main shared species were Chauliodus sloani, Ceratoscopelus warmingii and Diaphus garmani. The Jaccard species similarity index shows that the species similarity of mesopelagic fish was moderately different inside and outside the cold seep (I=35.8%). The average taxonomic distinctness (Δ⁺) of mesopelagic fish was significantly higher inside the cold seep than outside (P<0.01), but the variation in the taxonomic distinctness (Λ⁺) of mesopelagic fish community was inverse (P<0.01). From the perspective of pelagic distribution, there were significant differences in the species composition of the mesopelagic fish communities inside and outside the cold seep (P<0.01). In the surface waters, the mesopelagic fish diversity inside the cold seep was lower than outside, while it's inverse in the deeper water layers. There was a correlation between the fish communities at different stations inside and outside the cold seep in terms of water stratigraphy and timing, with higher similarity between the deeper layers inside the cold seep; and higher similarity between the nocturnal surface layers inside the cold seep area and the nocturnal surface layers outside the cold seep. The results show that there are differences in the mesopelagic fish communities inside and outside the cold seep, with more species of mesopelagic fish inside the cold seep and a higher diversity of species in the deeper water layers. Eight key species that distinguish the mesopelagic fish communities in the waters inside and outside the cold seep include Bolinichthys longipes and Vinciguerria nimbara, having a cumulative contribution of 56.06%.