Abstract: To investigate the characteristics of the phytoplankton functional group and its relationship with environmental factors in the West Lake of Eryuan County, Dali City, Yunnan Province, phytoplankton samples and water environment factors were collected monthly from September 2019 to August 2020. The modified Carlson trophic state index and the phytoplankton functional group (FG) classification were used to evaluate the degree of eutrophication in the water column of the lake and to explore the seasonal succession characteristics of phytoplankton. The preliminary results showed that: 1) Eryuan West Lake was eutrophic water with low transparency and turbidity. 2) Seven phyla and 72 genera of phytoplankton were identified by microscopic examination during the study, which were divided into 23 phytoplankton functional groups. MP (Oscillatoria sp.), S1 (Anabaenopsis sp.), H1 (Dolichospermum sp.) and SN (Cylindrospermopsis sp.) were the four dominant functional groups, all of which were filamentous algae of cyanobacterial phylum. Phytoplankton in autumn was mainly dominated by Oscillatoria sp. and Cylindrospermopsis sp., while it was mainly composed of Anabaenopsis sp. and Oscillatoria sp. in winter and spring, and mainly Oscillatoria sp. and Dolichospermum sp. in summer. 3) The results of Redundancy analysis showed that the key environmental factors for phytoplankton functional group succession in Eryuan West Lake were water temperature, pH, PO₄ ³⁻-P and TDP, especially the change of soluble phosphorus concentration contributed to the seasonal succession of the dominant functional group. Therefore, filamentous cyanobacteria were dominant in the eutrophication state of Eryuan West Lake, with a homogeneous algal structure.