Abstract: In order to understand the distribution of planktonic dinoflagellate in the coastal waters of Sanya Bay and its relationship with water environmental factors, samples of planktonic dinoflagellate and water environment in the coastal waters of Sanya Bay were collected in 2022 during dry season (April), wet season (July) and normal season (November). The results of sample analysis showed that, 50 species of dinoflagellates were identified in 5 orders, 13 families, 17 genera, including 25 species of red tide dinoflagellates, of which 13 species were toxic. There were 10 dominant dinoflagellates in the three water stages, of which 9 were red tide dinoflagellates and 5 were toxic red tide dinoflagellates. The first dominant species of dinoflagellates were Karenia sp., Peridinium quadridentatum and Heterocapsa circularisquama in dry season, wet season and normal season, respectively. The fluctuation of water environment factors in different water periods of the same site was small, and the content of inorganic nitrogen and phosphorus was high at both ends of the site and low at the middle site. Spearman correlation analysis showed that, NH₄⁺-N, NO₃⁻-N and NO₂⁻-N were the main driving factors affecting the community structure of planktonic dinoflagellate in the coastal waters of Sanya Bay. The results of RDA analysis showed that, the distribution of dominant red tide dinoflagellate in the coastal waters of Sanya Bay was significantly correlated with water temperature, salinity, inorganic nitrogen and phosphorus content.In general, the planktonic dinoflagellates, especially the dominant red tide dinoflagellates, responded most closely to inorganic nitrogen and phosphorus nutrients. The results of this study could be used as a supplement to the statistical data on the distribution of planktonic dinoflagellate species in Sanya Bay, and provide basic data for analyzing the correlation between dinoflagellate species and water environmental factors and monitoring the risk of red tide in this sea area.