To quantify the fish assemblage and its historical change, we conducted surveys in LongyangxiaReservoir in May, September, and December 2023 and analyzes the data against historical data. Nineteen speciesof fish were collected, belonging to 4 orders, 6 families, and 15 genera, among which Cyprinidae was thedominant family, accounting for 52.63% of the total species captured. The dominant species of fish inLongyangxia Reservoir were Carassius auratus, Oncorhynchus mykiss, Triplophysa scleroptera, Pseudorasboraparva, Hypomesus olidus, and Gymnocypris eckloni, and the dominant species of fish in the main stream of theYellow River were Gymnocypris eckloni, Schizopygopsis pylzovi, and Gymnodiptychus pachycheilus. The speciesdiversity index was highest in May and lower in September and December. ABC curve analysis showed that the Wvalue was greater than 0, the biomass dominance curve was above the abundance dominance curve, and thecommunity structure of Longyangxia Reservoir was relatively stable. Comparative analysis with data from 1981and 2005–2007 showed that, since the impoundment of Longyangxia Reservoir, indigenous fish decreased by twospecies and exotic fish increased by 10 species. Currently, Longyangxia Reservoir is dominated by fishes thatprefer quiet and slow water, are omnivorous, lower-middle, sedentary, exhibit sticky egg-laying, and the lowermouth. The ecological matrix of individual fish showed that the indigenous fish in Longyangxia Reservoir weremainly in the lower middle layer (R=0.556) and the lower mouth (R=0.889), which provided ecological nichevacancies for broadly adapted exotic fishes. The fish composition of Longyangxia Reservoir in 2023 showed asimilarity index of 0.33 (moderately dissimilar) with that of 1981, and a similarity index of 0.50 (moderatelydissimilar) with that of 2005–2007. The fish community structure changed significantly after the impoundment ofLongyangxia Reservoir in 1987. The analysis confirmed that the reservoir impoundment and the introduction ofexotic fishes were the main reasons for the changes in the fish community structure of the Longyangxia Reservoir.