Abstract: To reveal the seasonal variation characteristics of fish community structure and diversity in the lower reaches of the Songhua River, and to provide scientific basis for the protection, restoration and rational management of its fish resources, systematic field sampling and surveys were conducted at representative sampling sites in spring, summer and autumn from 2020 to 2022. Common freshwater fish sampling equipment was adopted during the survey to ensure the comprehensiveness and representativeness of the collected samples, thus laying a solid foundation for subsequent data analysis. A total of 66 fish species were collected through three-year continuous sampling, belonging to 8 orders and 17 families. The fish community in this region was mainly composed of benthic, sedentary and omnivorous fish, among which Cyprinidae species were the absolute dominant group, accounting for 59.09% of the total species number. There were 9 dominant species in the community throughout the year, and their total biomass accounted for 51.84% of the total biomass of all collected fish, indicating a certain degree of dominance in the overall community structure. There were obvious seasonal differences in the composition of dominant species in the fish community. In spring, the dominant species were Hemiculter leucisculus, Xenocypris argentea, Rhodeus sericeus and Carassius auratus, and their combined biomass accounted for 45.50% of the total biomass in spring. In summer, affected by seasonal environmental changes, the dominant species changed to C. auratus, X. argentea, Pseudorasbora parva and Silurus asotus, contributing 44.59% to the total summer biomass. In autumn, the dominant species increased to 6, including Hemiculter bleekeri, H. leucisculus, X. argentea, Acheilognathus macropterus, Cyprinus carpio and Carassius auratus, whose total biomass accounted for 65.42% of the total autumn biomass. Four common biodiversity indices were calculated to quantify the seasonal changes of the fish community, including Margalef richness index, Simpson dominance index, Shannon-Wiener diversity index and Pielou evenness index. The seasonal average values of these four indices were 3.9, 0.8, 2.2 and 0.7 respectively, which comprehensively reflected the seasonal differences of species richness, dominance and evenness in the fish community. Comprehensive analysis showed that the fish community in the lower reaches of the Songhua River was dominated by small and medium-sized sedentary, omnivorous and benthic fish, with Cyprinidae as the core component. Compared with the historical survey data of the same region, the total number of fish species and biomass in this survey were generally lower, which may be related to long-term anthropogenic disturbances such as overfishing. The species richness and diversity of the community were significantly affected by seasonal changes: moderate disturbance occurred in summer, while the community structure remained relatively stable in spring and autumn. Notably, Xenocypris argentea and Carassius auratus were dominant species in all three seasons, showing strong environmental adaptability in this region. To promote the sustainable and healthy development of the fish community in the lower reaches of the Songhua River, targeted management measures should be formulated and implemented according to its seasonal ecological characteristics. In particular, strict restrictions on overfishing should be enforced in summer to reduce anthropogenic disturbance, so as to maintain the stability of the fish community structure and improve its ecological function.