Investigation on the zooplankton of Lianhuadang River System which feeding into the Taihu Lake in Yixing in four seasons within a year were carried out in 2015. Total of 136 species of zooplankton were identified including 72 species of Protozoa, 39 species of Rotifer, 13 species of Cladocera, 9 species of Copepoda and 3 unkown species by the larvae. Community structure was mainly made up of small zooplankton, including Protozoa and Rotifer. The annual average density, biomass and biodiversity index were 14 664 ind/L (Table 4), 4.903 mg/L (Table 5) and 2.26 (Fig. 4) respectively. Annual average density and biomass reached to the largest at sampling site H (32 909 ind/L) and site 9 (12.176 mg/L) (Fig. 2, Fig. 3) respectively. The density and biological amount of zooplankton varied obviously with seasons. The quantity of species found in autumn was 65 species which is the highest in the four seasons (Table 2). Total of 20 species was recognized as dominance species in the samples collected in one or more than one seasons, however, none of them appeared in the samples through out the whole year. The main dominant species were Tintionnopsis sp., Difflugia sp., Euplotes sp., Keratella sp., Brachionus sp., Polyarthra spp., Bosmina sp., Sinocalanus dorrii and Mesocyclops sp. (Table 3). The total nitrogen, ammonia nirtogen and dissolved oxygen were the significant factors on zooplankton density (Table 6). The evaluation based on the biological diversity index and dominant species indicated clearly that the water quality of Lianhuadang River System was moderately polluted and meso-eutrophic.