Sanjiao Lake is a representative small urban lake in the middle and upper reaches of the Yangtze River. Eutrophication has became a problem for this lake recent years. In order to understand the seasonal community structure of planktonic crustacean and environmental variables, we made a monthly survey in Sanjiao Lake from July 2014 to June 2015 (Fig. 1) to provide a basis for managing eutrophication and providing supports for the ecological restoration efforts. We got the following data: water temperature varied seasonally 10℃ in winter and 28℃ in summer; sechi disk depth 0.27 to 0.72 m; total nitrogen and total phosphorous 1.50 to 3.34 and from 0.07 to 0.58, respectively (Table 1). We identified a total of 22 species, belonging to 17 genera and 9 families of planktonic crustacean from the seasonal samples. The community included 13 species in Cladocera and 9 species in Copepoda (Table 2). The species composition showed a light similarity between summer and autumn or winter with Jaccard index ranging from 0.45 to 0.50, and a medium similarity among other seasons with Jaccard index ranging from 0.60 to 0.72 (Table 3). The density of planktonic crustacean ranged from 0.6 to 13.1 ind/L with an average density of 3.7 ± 3.6 ind/L. The lower density occurred in summer and autumn, while the peak was in February. Copepoda and nauplii dominated the community in most months except May and June, accounting for 55.2%﹣95.4% of the total density (Fig. 2). The dominant species were Cyclops vicinus, Microcyclops leuckarti, Moina irrasa, Microclops varicaricans, M. rectirostris, Diaphanosoma leuchtenbergianum and Bosmina longirostris (Fig. 3). Species richness of planktonic crustacean ranged from 6 to 15, with an average of 10 ± 3. Higher species richness occurred from late autumn to spring, while the lower value occurred in summer. Simpson index of the community ranged from 0.76 to 0.90, with an average of 0.80 ± 0.04 (Fig. 4). Pearson correlation analysis showed that the total density of planktonic crustacean had a significantly negative relationship with the water depth (r =﹣0.636, P < 0.05, n = 11), reflecting that seasonal fluctuation of water level affect the density of planktonic crustacean. Canonical correspondence analysis (CCA) indicated that water temperature and sechi disk depth were the key factors affecting the seasonal variation of species composition of planktonic crustacean (P < 0.05, Fig. 5). It is inferred that predator pressure from the intensive aquaculture for Silver Carp (Hypophthalmichthys molitrix) and Bighead Carp (Aristichthys nobilis) was likely responsible for the low density of planktonic crustacean community in Sanjiao Lake.