Zooplankton are important food organisms in the marine ecosystem, and their community structure and distribution reflect the productivity of the waters. To investigate the zooplankton structure and the environmental factors affecting ecological niche differentiation in the waters of Tahe Bay, Lushun, a survey was conducted in September 2021 and March, April, and November 2022 in the waters of Tahe Bay. The results showed that there are 31 species representing four phyla, with an annual mean abundance of 12.42 × 10³ ind/m³, dominated by Copepoda (13 species, 41.94%), with zooplankton richness indices ranging from 0.83 to 2.44, diversity indices ranging from 0.84 to 2.42, and evenness indices ranging from 0.14 to 0.84. Pearson’s correlation of zooplankton abundance and community diversity with environmental factors, such as water temperature, salinity, dissolved oxygen, NH₃-N, and NO₃-N, was significant in the waters of Tahe Bay. There were 13 dominant species, mainly consisting of broad-niche species, among which Oithona similis was the dominant species in all four seasons; the degree of niche overlap of the dominant species was related to the seasons, with a serious niche overlap accounting for 81.0% in September 2021, and there was no niche overlap in March 2022; 33.3% of the species had a severe ecological niche overlap in April 2022, with a serious niche overlap accounting for 86.7% in November 2022. The results of redundancy analysis (RDA) indicated that water temperature, salinity, DO, and DIP are the main environmental factors affecting the ecological niche differentiation of the dominant species of zooplankton.