Abstract: In order to investigate the characteristics of fish resources in the middle reaches of the Jinsha River, surveys were conducted in the mainstreams and its tributaries in December 2021 (dry season), March—April 2022 (normal season), and July—August 2022 (wet season). The results showed a total of 47 fish species, belonging to 3 orders, 9 families, and 34 genera, Among them, 8 species were classified as national second level key protected species, and 14 were identified as alian species. This study analyzed the species composition, ecological types, community similarity, and biodiversity of fish. The mainstream of the middle reaches of the Jinsha River can be divided into three groups: the Liyuan-Longkaikou section, the Ludila-Guanyinyan section, and the Jinsha-Yinjiang section. Significant differences in fish communities were observed in these groups (P<0.05). The Liyuan, Ahai, Jinanqiao, and Longkaikou sections predominantly featured indigenous hydrophilic fish, with dominant species such as Schizothorax chongi, Schizothorax wangchiachii, and Coreius guichenoti. The Rudila and Guanyinyan sections are mainly composed of indigenous riffle fish and alian Eurytopic fish, with dominant species including Carassius auratus auratus, Hemiculter leucisculus, and Schizothorax wangchiachii. The Jinsha and Yinjiang river sections exhibited a fish composition primarily composed of small, exotic, calm water and Eurytopic species, with dominant species like Pseudorasbora parva and Rhodeus sinensis. Compared with the historical data, a trend of miniaturization in fish and a transition from plateau flora to plain flora in the middle reaches of the Jinsha River were observed. This study supplements the basic data on the current status of fish resources and proposes protection suggestions, aiming to provide a scientific basis for the management and protection of fish resources in the specified section of the river.