In order to provide basic data for the oncoming bio-manipulation based on fish and reference system for hereafter investigating its subsequent effect on fish composition in a newly established water supply reservoir (Qingcaosha Reservoir in Shanghai of China), fish assemblages in this reservoir were sampled by multimesh gillnets from Jul. to Oct. in 2010. A total of 993 individuals, belonging to 18 species, representatives of 5 orders, were caught, of which Cypriniformes accounted for 72.2% of the total species number. Based on importance value, Hemiculter bleekeri, Coilia nasus, Carassius auratus and Pseudobagrus nitidus were the dominant species. Clustering analysis (CA) showed that fish assemblages significantly differed spatially, and there were marked effects of net size on species composition of fish catch. One-way ANOVA indicated that there were no significant spatial differences in fish diversities. Although Shannon’s species diverisity (H^’) was conspicuously positively correlated with Margalef’s species richness (R) and evenness (J), respectively, H^’was governed more by R. Invertivorous (44.4%) and omnivorous (33.3%) fishes prevailed in species number among the 4 feeding functional groups, and sedentary fishes (77.8%) predominated among the 4 ecological groups. Contrasting to the adjacent estuarine zone with more species of Perciformes and dominated by estuarine and/or inshore fishes, Qingcaosha Reservoir was characterized by dominance of freshwater or low salinity water fishes. It is presumed that isolation and decreased salinity that resulted from reservoir construction would be responsible for the fish characteristics of Qingcaosha Reservoir.