Abstract: To investigate the trophic structure of fish communities and utilization of fish food resources in valley-type reservoirs, the characteristics of fish community structure in the Weishui Reservoir (Hubei Province) were studied in summer and autumn 2020 based on stable carbon (δ¹³C) and nitrogen (δ¹⁵N) techniques. Moreover, the contributions of four potential carbon sources (particulate organic matter, terrestrial plants, epilithic algae, and organic detritus) to fish with different feeding habits were evaluated by a Bayesian stable isotope mixing model. The results showed that there were seasonal differences in the trophic structure of fish communities in the Weishui Reservoir. The Layman’s community-wide metrics showed that the δ¹⁵N range (NR), δ¹³C range (CR), total area (TA), mean nearest neighbor distance (MNND), standard deviation of nearest neighbor distance (SDNND) and standard ellipse area (SEA) of fish communities in summer were greater than those in autumn, suggesting a higher diversity of food sources, lower degree of trophic redundancy, and more complex and stable food web structure in summer. Fish relied on allochthonous carbon in summer and autumn, but significant differences between those in two seasons (P<0.05) were found. Overall, terrestrial plants made the greatest contribution to the carbon sources of fish in the Weishui Reservoir, however, epilithic algae and organic detritus were the secondary important carbon sources in summer and autumn, respectively. Particulate organic matter was unimportant for fish at two seasons. In summer and autumn, detritivorous fish Xenocypris davidi exhibited the highest reliance on allochthonous carbon, whereas filter-feeding fish (bighead carp) and herbivorous fish (blunt snout bream) showed the lowest reliance on allochthonous carbon. Trophic positions of fish ranged between 2.5—3.5 (averaged 3.01) in summer, whereas between 2.9—3.8 (averaged 3.32) in autumn. For the nine common species at two seasons, piscivorous fish (Siniperca chuatsi and Erythroculter mongolicus), omnivorous fish (Cyprinus carpio and Carassius auratus) and detritivorous fish (Xenocypris davidi) showed great variations in TPs. Based on the analysis of stable carbon and nitrogen isotopes for fish with different feeding habits, we found that TPs of fish in autumn were significantly higher than those in summer (P<0.05). Our study revealed a significant difference between the utilization of carbon sources by fish between summer and autumn, with TPs of fish in autumn was generally higher than that in summer. This is primarily ascribed to the low water levels of the Weishui Reservoir in autumn because low water-levels do not allow the entry of terrestrial plants into aquatic food webs, thereby resulting in intensified competition of fish for food resources and predating. Analysis of the Bayesian stable isotope mixing models suggests that of herbivorous and omnivorous fries such as grass carp and blunt snout bream are recommended to release to the reservoir in early spring (February-March), while detritivorous fries are recommended for release in autumn and winter.