Abstract: To investigate phytoplankton species composition and biodiversity in Jiulongjiang Estuary Mangrove Nature Reserve (JEMNR), seasonal sampling of phytoplankton was carried out at 6 stations from January to October in 2016. The trophic state of the water in the reserve was assessed with Shannon-Wiener diversity index, Pielou evenness index and Margalef abundance index. A total of 138 phytoplankton species in 91 genera and 5 phyla had been identified in this mangrove wetland. Bacillariophyta dominated the phytoplankton community, with 50.00% of the total species, followed by Chlorophyta (31.16%), Pyrrophyta (8.70%), Cyanophyta (5.07%) and Euglenophyta (5.07%). The dominant species were Melosira sulcata, Skeletonema costatum, Cyclotella stratia, Navicula sp., Synedra robusta and Nitzschia sp. Phytoplankton density ranged from 0.82×10⁵ to 4.55×10⁵ cells/dm³, with average value of 1.97×10⁵ cells/dm³. In contrast to the remarkable seasonal and spatial variation of species compositions, the phytoplankton quantity was changed little with seasons though they varied spatially with sites. The diversity indexes of Shannon Wiener, Margalef and Pielou were 2.10～3.55, 1.05～3.61, 0.78～0.95, respectively. According to the evaluation on the these index, it seemed that the water in mangrove wetland was not severely polluted. It was necessary to strictly control the wastewater and effluent discharging and inland run off into the mangrove.