To explore the fish species composition, biodiversity and spatial and temporal characteristics of fish assemblages for a water supply reservoir, Qingcaosha reservoir, the background survey was operated in three regions (western waters, control waters and stocking waters) of Qingcaosha waters and there were four sampling sites in each region in May, July, October and December in 2011. A total of 22 species were collected, belonging to 7 families, 5 orders, and Cyprinidae was the most abundant. The community was composed of three ecological types: freshwater (17 species), estuarine (3 species), diadromous(2 species), and freshwater species were absolutely dominant. In all species, the dominant species were Coilia ectenes, Carassius auratus, Cyprinus carpio and Pelteobagrus nitidus. The highest diversity and richness region was western waters. The diversity of spring was the highest, and that of winter was the lowest. Cluster analysis showed that there were significant differences in the fish communities among the sampling sites and between seasons. Reservoir construction and decreased salinity resulted in the fact that the fish diversity decreased and the small-size fishes became dominant. so it is suggested that the delivery of large-size fishes should be increased in the activity of fishery enhancement and release.