Abstract: The Nansha Reservoir in Yunnan Province is a key hydraulic project in the cascade development of the Red River Basin, but the response of its fish resources to seasonal environmental changes in the reservoir environment remains unclear. This study aims to clarify the dynamic characteristics of fish resources during the breeding season (May) and the overwintering preparation period (October), thereby providing a scientific basis for the reservoir's ecological management, exotic species control, and native fish conservation. In May and October 2024, we integrated hydroacoustic surveys with traditional fishing methods to systematically analyze the spatiotemporal variations in species composition, body length structure, and spatial distribution of the fish community. The results show that: 1) Regarding species composition, 26 fish species were identified in May, with Cypriniformes dominating in both relative abundance (70.92%) and biomass (55.93%). In October, 43 species were identified; the relative abundance of Cypriniformes decreased to 60.44%, while Siluriformes increased significantly in both abundance (25.26%) and biomass proportion (31.19%). The total biomass proportion of non-native species reached 28.19%. 2) Regarding body size structure, target strength (TS) inversion indicated that the proportion of small individuals (<10 cm) was notably high in May (78.81%) but decreased significantly to 61.43% in October. Concurrently, the proportion of medium-sized individuals (10–30 cm) increased substantially to 30.46%. 3) Regarding spatial distribution, high-density areas in May were concentrated in shallow waters from the reservoir embayment to the tail area Average density: (0.2065±0.1858) ind·m⁻³, whereas in October, they shifted to the transition zone between the central reservoir and the embayment (0.1940±0.1720) ind·m⁻³, with enhanced spatial continuity. Vertically, fish were primarily concentrated in water layers ≤10 m during the breeding season (74.88%) and showed significant descent during the overwintering preparation period. The study is the first to systematically reveal the coordinated seasonal shifts in the Nansha Reservoir fish community, characterized by "increased species richness, succession of dominant taxa, growth of juvenile cohorts, and distributional descent". Meanwhile, it is found that exotic species have formed an ecological niche pattern of "low abundance–high biomass".