Abstract: Based on data obtained from zooplankton surveys conducted in nearshore waters of northwest Hainan Island, in November (autumn) of 2016, February(winter), May(spring) and August(summer) of 2017，the community structure of zooplankton was studied as well as the species composition, dominant species and its ecological groups, biomass and abundance. As result, a total of 215 species in 113 groups of 7 phyla were recored. The species number identified in autumn was the highest (134) and it was very similar in winter (113) and spring (111). The species number found in summer was the lowest (94). Copepoda and Hydrozoa were the most dominant groups. The spacial distribution of zooplankton species was higher in inshore and offshore than in the middle waters in autumn and winter, while it gradually increased from the inshore to offshore waters in spring and summer. The numbers of dominant species changed slightly with seasons except for an obvious succession of dominant species. The common dominant species occurred in 4 seasons is only Subeucalanus subcrassus and Sagitta enflatafor. The dominant species can be divided into 5 ecological groups, included warm water species, euruthermous and euryhaline species, nearshore warm water species, tropical warm water species and cosmopolitan warm water species. The average abundances of zooplankton in autumn (124.75 ind./m3) and winter (152.43 ind./m3) were similar while they were much lower in spring (64.76 ind./m3) and summer (74.44 ind./m3). The horizontal distribution of zooplankton abundance showed higher in the nearshore waters and lower in the offshore waters in autumn and winter. However, the average abundance was slightly higher in winter than autumn in waters deeper than 20 m. In contrast, the abundance was lower in the nearshore waters and it was higher in the offshore in spring and summer. The average abundance was obviously lower in spring and summer than those in winter and autumn. The average biomass of zooplankton was the highest in winter (263.68 mg/m3), the next was in autumn (147.38 mg/m3), summer (61.45 mg/m3) and spring (59.13 mg/m3) in consequence. The distribution pattern of biomass was similar to the abundance distribution.