Qingcaosha Reservoir, located in the Yangtze Estuary, is the largest estuary reservoir built and was put into use at the end of 2010. In order to grasp the trend of fish community structure changes after the completion of Qingcaosha Reservoir, a five-year fish survey was conducted quarterly from January 2015 to December 2019. The results show that 5 158 fish samples were collected and 51 species were identified, belonging to 9 orders 16 families 41 genera; Cyprinids account for the highest proportion, 87.1%; the proportion of migratory fish, anadromous migratory fish, estuarine fish decreased year by year from 2015 to 2019, and the proportion of freshwater settled fish increased year by year; silver carp, bighead carp and crucian carp are dominant species in all survey years, and the dominance of crucian carp is increasing year by year; Cluster clustering analusis results show that the community structure was similar in 2015 and 2016, and similar in 2017, 2018 and 2019. Combined with the annual changes of Margalef, Pielou and Shannon Wiener index values, it shows that the disturbance degree of the community structure of Qingcaosha Reservoir is on the rise. At present, freshwater fishes are dominant in Qingcaosha Reservoir, and migratory and estuarine fishes gradually disappear from the reservoir. The fish composition is getting closer and closer to the characteristics of freshwater lakes in the lower reaches of the Yangtze River, which is different from the fish community structure in the peripheral Yangtze River estuary.