Abstract: The abundance and spatial variation of net-phytoplankton were studied in Wanshan islands adjacent sea, based on field survey in May 2014.We analyzed the environmental factors, species composition and cell density of net-phytoplankton (mesh size 77 μm), aimed to understand the impact of water exchange on community structure.A total of 93 species of phytoplankton were identified.Bacillariophyta was the dominant functional group, represented the highest number of species, accounting for 73.1% of total abundance.There were significant effects between net-phytoplankton community structure and environmental factors such as water temperature, salinity, chemical oxygen demand, dissolved oxygen, pH, chlorophyll a and N:P. In the mixed area, Pearl river plume and oceanic water converged to form salinity fronts, the number of euryhaline species, offshore species and oceanic species were higher than other areas.Net-phytoplankton abundance was represented by euryhaline species in the mixed area, dominated by small particle size species of Bacillariophyta and the dominant species was Skeletonema costatum, with more than 90.2% of total abundance.Mean net-phytoplankton abundance was 5.96×10⁸/m³ in the mixed area, about 4000 times higher than other areas.Bacillariophyta was still the most abundant group in the low salt area, accounting up to 80.6% total abundance, but the proportion of big particle size species of Bacillariophyta such as Bellerochea malleus, Coscinodiscus gigas, Hemidiscus hardmannianus and Planktoniella formosa increased obviously.Moreover, the proportion of big particle size species of Pyrrophyta such as Ceratium breve, C.fusus, C.macroceros and Noctiluca scintillans increased in the high salt area, and Pyrrophyta further increased to 40.7%, where Bacillariophyta and Pyrrophyta became the dominant group together.The superiority of Skeletonema costatum is particularly prominent, together high phytoplankton abundance, support high primary production in Wanshan islands adjacent sea.