Abstract: Poyang Lake is a large Yangtze-connected lake in the middle and lower reaches of the Yangtze River and its water level fluctuates periodically with the hydrological rhythm. In order to clarify the characteristics and influencing factors of plankton community structure during different hydrological periods, the survey on plankton structure was conducted at five typical sampling sites of Poyang Lake in August (wet season) and December (dry season) of 2017. The results indicated that phytoplankton assemblage was composed of 186 species and 75 genera, belonging to 8 phylum. Bacillariophyta and Chlorophyta dominated in wet and dry seasons. Zooplankton was composed of 76 species, belonging to four taxonomic groups. Protozoa and Rotifers were dominant during wet and dry seasons. The density and biomass of phytoplankton were statistically significantly higher during wet season than those during dry season (P<0.01). The density of zooplankton in wet period was higher than that in dry water period, but there was no significant difference (P>0.05). The biomass of zooplankton (P<0.05) was significantly different in different hydrological periods. Redundancy analysis suggested that there was negative correlation between the plankton community structure and transparency. The conductivity showed a positive correlation with the plankton community structure. Transparency, conductivity and nutrient were key factors affecting the community structure of plankton during wet season, whereas water temperature and dissolved oxygen were key factors affecting the community structure of plankton during dry season. Based on the diversity indices of Shannon-wiener, Margalef and Pielou, the results showed that the water quality of Poyang Lake was at low-to-medium pollution level. Our findings revealed the impact of two hydrologic periods on the plankton of Yangtze-connected lakes. Seasonal change did not change the species composition and dominant species of plankton in Poyang Lake, but affected the abundance and diversity of plankton greatly.