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 both planktonic dinoflagellate and water environment in the coastal waters of Sanya Bay were collected and investigated during the dry season (April), wet season (July) and normal season (November) 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 and 17 genera, including 25 species of red tide dinoflagellates, of which 13 species were toxic. There were 10 dominant dinoflagellates in the water of seasons, of which 9 were red tide dinoflagellates and 5 were toxic red tide dinoflagellates. The most dominant species of dinoflagellates were Karenia sp., Peridinium quadridentatum and Heterocapsa circularisquama in dry season, wet season and normal season, respectively. The fluctuations of water environment factors at the same location in the different water seasons were small, and the concentrations of inorganic nitrogen and phosphorus were high at both ends of the Sanya Bay and low at central Sanya Bay. 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. In general, 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 both analyzing the correlation between dinoflagellate species and water environmental factors and monitoring the risk of red tide in this sea area.