Understanding the spatiotemporal dynamics of fish communities and their associations with environmental gradients in large reservoirs is of great significance for formulating scientific fisheries resource conservation and management strategies and optimizing hydrological operations in cascade reservoirs at the watershed scale. This study focused on the Nuozhadu Reservoir, a key control project on the main stem of the Lancang River. Quarterly fish surveys were conducted at 11 sampling sites in July, October, and December 2024 and April 2025, combined with water environmental factors, to systematically examine fish community structure, α-diversity patterns, and their environmental associations after reservoir impoundment. A total of 43 fish species were recorded, with Cypriniformes as the dominant group. Non-native species (e.g., Hemiculter leucisculus, Parachromis managuensis, and Coptodon zillii) consistently dominated throughout the year, whereas local endemic species mostly occurred as marginal components of the community, with low abundance and low index of relative importance (IRI). From the perspective of species composition, the fish community was characterized by a high proportion of resident (76.74%), lentic (58.14%), and omnivorous (58.14%) species. Fish α-diversity remained overall stable over time but exhibited pronounced spatial differentiation along the longitudinal gradient from near-dam to far-dam areas, with significantly higher diversity in the reservoir tail than in the reservoir head. Joint analyses using generalized linear models (GLM) and generalized additive models (GAM) revealed that fish diversity was statistically associated with multiple environmental variables, among which distance from the dam had relatively high explanatory power and was positively associated with fish diversity. In addition, fish diversity was also statistically associated with dissolved oxygen, total nitrogen (increasing first and then decreasing), and water depth (decreasing). This study systematically revealed the structural characteristics, spatiotemporal dynamics, and environmental driving associations of fish communities in the Nuozhadu Reservoir after impoundment. These findings provide a scientific basis for the conservation and management of fishery resources in large reservoirs in China.