Abstract: To provide scientific basis for the fishery management, the habitat restoration of rare and endangered fish, and the aquatic ecological environment protection in Lugu Lake, we investigated the biodiversity, density and biomass of plankton in Lugu Lake in January, May and September of 2020 by using the systematic sampling method. Meanwhile, we studied the vertical variation of plankton density. A total of 148 species of phytoplankton had been identified, which belonged to 7 Phyla, 10 Classes, 23 Orders, 41 Families and 85 Genera, among which Mougeotia parvula and Fragilaria crotonensis were the main dominant phytoplanktons. A total of 95 species of zooplankton had been identified, which belonged to 12 Orders, 28 Families and 58 Genera, among which Daphnia hyalina and Ceriodaphnia quadrangula were the main dominant zooplanktons. The species composi tion of plankton in Lugu Lake had changed to some extent due to water quality change and human disturbance. The phytoplankton Shannon-Wiener index (H') and Pielou evenness index (J') were consistent in the sampling months, with a descending order of January>May >September. The density and biomass of plankton were significantly different in the sampling months, and the density of phytoplankton followed a descending order of September>May>January. The density of zooplankton varied little in the sampling months. The phytoplankton biomass followed a descending order of January>September>May. The zooplankton biomass followed a descending order of May>September>January. The results of correlation analysis show that in May and September, there were significant positive correlations in the density and biomass between species with close predation relationship. The vertical variation of plankton in Lugu Lake shows that the vertical variation of plankton was related to the appearance and movement of thermodynamic stratification in the lake. With the increase of water depth: 1) The density of phytoplankton first increased and then decreased in January and May, but first decreased then increased in September. 2) The density of zooplankton decreased in January, first increased and then decreased in May, but decreased and then increased in September. The zooplankton-phytoplankton relationship also varied at different water layers. It is preliminarily assumed that the vertical change of temperature affects the vertical distribution of plankton, and their relationships in different water layers are also affected.