Abstract: Zooplankton, as primary consumers, plays an important role in the ecological process of material transformation, energy flow, and information transmission. It is one of the important indicators for evaluating the service functions of water ecosystems. A total of 70 species of zooplankton were found in Erhai Lake in 2019, including 43 species of rotifers, 19 species of cladocerans and 8 species of copepods. The dominant species were spiral tortoise shell rotifer and Daphnia Magna. Spiral tortoise beetle rotifer was the dominant species in spring, summer and winter, and Daphnia Magna was the dominant species in autumn. The density and biomass of zooplankton showed seasonal differences. The annual average value was 173.8 ind/L with significantly highest density in summer (218.17 ind/L) and the lowest density in autumn (103.81 ind/L). From 1957 to 2019, the density of rotifers, cladocerans and copepods of 2019 was the second lowest year in the Lake. Compared with previous years, the downward effect of zooplankton on algae is weakening, and the consumption of organic debris in the lake is weakening.