Abstract: To investigate the short-term responses of plankton communities to thermal discharge from Qinshan nuclear power plant (Qinshan NPP), a continuous 24-hour survey of environmental factors (surface and bottom layers) and the abundance and community composition of zooplankton and phytoplankton was conducted at two sites (L1 and L2) near the drainage outlets of the plants (Phase I and Phase II expansion) on November 11 to 12, 2018. The results revealed that the water temperature exhibits a trend of being higher in the bottom layer than in the surface layer near the drainage outlet, whereas salinity shows the opposite vertical distribution, with surface values exceeding those at depth. Variations in salinity and dissolved silicate (DSi) align closely with tidal fluctuations. However, the trends in dissolved inorganic nitrogen (DIN) exhibited a different variation pattern: one shows a fluctuating decline, while the other remains relatively stable. Although the dominant species of plankton are consistent between the two stations, there are significant differences in the temporal variations of zooplankton and phytoplankton abundance, as well as in community composition. At L1, the diurnal variations in phytoplankton and zooplankton abundance were broadly inverse to each other, with phytoplankton abundance following a sequence of initial increase, subsequent decline, and eventual stabilization. At L2, by contrast, phytoplankton abundance remained at a relatively low level with no clear trend, while zooplankton exhibited a marked peak at 21:00, dominated by Limnoithona sp. and copepod larvae. Redundancy analysis (RDA) indicated that tide and salinity variations driven by tidal were the most significant environmental factors affecting the abundance of dominant phytoplankton species and zooplankton species near the two drainage outlets, while water temperature exerted only a comparatively minor influence.