Abstract: In May, July and September of 2019, composite gillnets and fixed nets were used to investigate the fish resources in the lakes and rivers of the Hulun Lake water system. The structure and species diversity of fish communities and their changes, and the ways to protect the species diversity of fish communities were discussed in this paper. The results showed that 31 fish species in 5 orders and 7 families have been recorded in the lakes and rivers of Hulun Lake water system. Among them, the numbers of species of Cypriniformes and Cyprinidae were greater, which were 26 species and 23 species, respectively. The dominant species in the fish communities all were settled species. The 22 fish species have been recorded in Hulun Lake, and the dominant species were Hemiculter bleekeri bleekeri, Culterichthys enythropterus, Carassius auratus gibelio, Cyprinus (Cyprinus) carpio and Leucicus waleckii. The Shannon-Wiener diversity index, Margalef richness index and Pielou evenness index of the fish communities all were smaller in Hulun Lake, with the apparent trend of individual fish miniaturization. It is recommended to strengthen the water regulation of the rivers entering Hulun Lake to optimize the structure of fish community in Hulun Lake, to increase the reproduction and release of filter-feeding fish, to restore the resources of large benthic bivalves in Hulun Lake, and to carry out restoration of indigenous fish habitats.