Abstract: According to the bottom trawl survey data in southwestern continent shelf of Nansha area in different seasons of 2013, we analyzed the species composition and biodiversity indices. We collected 251 species, including 202 fish species, 16 cephalopod species and 33 crustacean species. The total number of species of catch composition was the highest in summer and the lowest in winter. According to the index of relative importance ( IRI) , the dominant species were Priacanthus macracanthus, Uroteuthis edulis and Saurida tumbil, whose IRIs were higher than 500; the main species were Nemipterus bathybius, Gemphylus serpens, Lophiomus setigerus, Trachinocephalus myops, Trichiurus lepturus, Saurida undosquamis, Decapterus macrosoma, and so on, whose IRIs were 100 ～500. Cluster analysis indicates that the dominant species structure was more similar between spring and autumn. The Margalef richness index ( D) reached the maximum in summer but the the the minimumin winter; the Shannon-Weiner diversity index ( H) reached the maximum in summer but the minimum in autumn; Pielou index ( J) reached the minimum in autumn, but there was little difference among the other three seasons; dominance index ( ) reached the maximum in autumn but the minimum in summer.