Abstract: In order to understand the composition of fish species diversity and assemblage structure in the Dianshan Lake and the upper reaches of Huangpu River, the quantitative and qualitative fish samples were collected from 10 sampling sections in June (summer) and October (Autumn) of 2020, and their species diversity and assemblage structure were analyzed. A total of 11 408 fish specimen were collected, comprising 49 species belonging to 9 orders, 14 families, and 37 genera. In summer and autumn, there were 29 and 30 species in the Dianshan Lake, 37 and 29 species in the upper reaches of the Huangpu River, respectively. The main dominant species were Coilia nasus、Carassius auratus and Acheilognathus macropterus. The average of Shannon Wiener diversity index (H’), Pielou evenness index (J’), Simpson dominance index (C) and Margale richness index (D) in autumn were higher than those in summer. The fish assemblage in the Dianshan Lake was highly similar to those in the upper reaches of the Huangpu River. The abundance-biomass comparison curve (ABC curve) revealed that fish assemblage was unstable, especially in the Dianshan Lake. The cluster analysis and non-metric multidimensional ordination indicated that the fish assemblage could be divided into three groups, e.g. The upper reaches of the Huangpu River group, the Dianfeng Ⅱ and the Dianshan Lake group Ⅲ. Pseudobrama simoni, Coilia nasus, Acheilognathus macropterus, Hemiculter leucisculus, Carassius auratus were the main discriminating species causing the differences among the three groups. Compared with previous survey data, the species of fishes showed an increasing trend. However, the fish assemblage in the Dianshan Lake and the upper reaches of the Huangpu River was still dominated by small-sized fishes, obviously disturbed by human activities. It is necessary to continue to improve water quality, carry out fish proliferation and release, reduce fishing pressure, and control the fishing area and time of shellfish bottom trawl, so as to improve stabilization of fish assemblage and protect the ecosystem health of the Dianshan Lake.