Abstract: The central and southern waters of the Nansha Islands is an important tropical marine ecosystem in China, and the distribution characteristics and environmental driving mechanisms of phytoplankton communities are the keys to understanding the ecological processes in this region. In this study, we set up 27 stations in this sea area in spring 2024, and analyzed the species composition, spatial pattern and their relationships with environmental factors by investigating the net-collected phytoplankton communities and related environmental parameters. The results show that a total of 192 species of phytoplankton belonging to 58 genera and 4 phyla were identified, among which Bacillariophyta (101 species) and Dinophyta (80 species) were dominant in terms of species number; the average total abundance of phytoplankton was 16 288 cells·m⁻³, and Cyanophyta had an absolute quantitative advantage, with its high-abundance areas concentrated in the Reed Bank and the central and western sea areas. Cluster analysis divided the stations into three types: Group I (Multi-element mixed type), Group II (Transitional type) and Group III (Cyanophyta absolute dominant type) At Group III stations with low nitrate concentrations, the dominance of Trichodesmium exceeded 92%. Redundancy analysis show that nitrate and dissolved oxygen were the key environmental factors affecting the spatial differentiation of the community, explaining 14.2% and 6.4% of the variation respectively. Diversity analysis indicates that there was significant spatial heterogeneity in the stability of phytoplankton communities in this sea area, with more complex community structure and higher stability in the southern open sea areas, while lower diversity in the northern shelf area.