In order to understand the fish community structure in Qingcaosha Reservoir in the Yangtze River estuary， the fish resources investigation and community structure analysis were carried out in 2021. A total of 3209 individuals were collected， identified and classified into 9 orders， 13 families ，36 genera and 46 species. The freshwater carnivorous fish species are the largest. The Index of Relative Importance，IRI of Hypophthalmichthys molitrix， Aristichthys nobilis， Hemibarbus maculatus， Carassius auratus， Coilia nasus， Culter alburnus， Saurogobio dumerili and Lateolabrax japonicus were more than 1 000 and all were dominant species.The niche analysis of dominant species showed that carnivorous and omnivorous fish had higher niche overlap in food resources and living environments. The population expansions of the two groups of fish should be more limited by food， space and other resources than the low niche overlap groups. The niche overlap among Silver carp， Bighead carp and other fish is lower， and the competitions for food， space and other living resources in the reservoir are also lower. In 2021， the Margalef species richness index（D） was 5.70， the Shannon-Wiener diversity index（H'） was 3.01， the Pielou evenness index（J） was 0.78， and the Simpson dominance index（C） was 0.81. Compared with the results of fish diversity and the community structure from 2015 to 2019 in the Reservoir ， the diversity index is increasing significantly. The ABC curve analysis showed that the community structure was less disturbed by external factors.