To understand the characteristics of the phytoplankton community structure and its relationship with environmental factors, an investigation was carried out monthly from September 2019 to August 2020 in Xinfengjiang Reservoir, a large oligotrophic reservoir in Southern China. The results showed that the water quality indexes of Xinfengjiang Reservoir met the CriterionⅠaccording to the "Surface Water Environmental Quality Standard" (GB 3838-2002). A total of 189 phytoplankton species were identified, belonging to 8 phyla and 89 genera. Among these species, 94 were members of Chlorophyta, 36 Cyanophyta, 33 Bacillariophyta, 10 Euglenophyta and 9 Pyrrophyta, accounting for 49.74%, 19.05%, 17.46%, 5.29% and 4.76% of the total phytoplankton species, respectively. The phytoplankton community structure was Chlorella-Diatom form in winter and spring 2020, and Chlorella-Cyanophyta form in summer and autumn 2019. The abundance of phytoplankton ranged from 0.33×10⁶ to 11.78×10⁶ cells/L with an average range of 3.75×10⁶ cells/L. it was higher in autumn and winter in 2019 than that of spring and summer in 2020. The phytoplankton biomass has a range of 0.26-2.49 mg/L with an average of 1.19 mg/L. The temporal trend of biomass was as similar as that of abundance with dominated groups of Cyanophyta and Chlorophyta as well as Chlorophyta and Diatoms Correlation and canonical correspondence analysis showed that water temperature was the key factor affecting phytoplankton community structure in Xinfengjiang Reservoir, followed by water transparency and electrical conductivity. The present study had great significance for further understanding the ecological characteristics of phytoplankton in the oligotrophic reservoir, as well as expanding the study scope of limnology.