To grasp the composition and diversity of the fish community in the Youshui River, the largest tributary of the Yuanjiang River, during the early stage of the ten-year fishing ban policy of the Yangtze River, fish resources were investigated at the Daxi Town monitoring section in the upper reaches and the Liangjiangkou section in the lower reaches during the summers and autumns from 2022 to 2023. The diversity index, similarity, dominant species, and ecological types of the community were analyzed. A total of 4,578 fish were counted, belonging to 52 species, 29 genera, 8 families, and 5 orders. The order Cypriniformes occupied the dominant position, with Cyprinidae being the dominant family. The dominant species included Saurogobio dabryi, Hemiculter tchangi, Paracanthobrama argentatus, Cyprinus carpio, Culter alburnus, Carassius auratus, and Hemiculterella sauvagei. Among them, S. dabryi maintained a stable dominant position in both summer and autumn, but the dominance of some economic fishes and small-to-medium-sized fishes showed dynamic differences between seasons. The diversity and richness in summer were slightly higher than those in autumn, while the evenness was higher in autumn. Furthermore, the diversity in the downstream section was higher than that in the upstream section. During the survey period, nine species of fish were listed in the China Biodiversity Red List, among which Coreosiniperca roulei was classified as Near Threatened (NT). The results showed that after the implementation of the ten-year fishing ban policy, the fish community diversity in the Youshui River was at a medium level in the early stage. Influenced by the habitat filtering of the cascade reservoirs, the fish community was dominated by small and medium-sized sedentary species, presenting a potential underlying niche competition and invasion risk from the exotic species Cirrhinus mrigala.