Abstract: To explore the trophic structure and food web structure of the fish community in the Xiaoxi'ergenqi River, an investigation of its fish resources was conducted in the summer (June) and autumn (September) of 2023. By analyzing the carbon and nitrogen stable isotope values (δ¹³C, δ¹⁵N) of various organisms, the SIBER and MixSIAR packages in R were used for relevant calculations and analysis to determine the seasonal differences in the trophic structure of the catch community, and a preliminary food web of the river was constructed. The results showed that the δ¹³C values of fish in summer ranged from (-36.53‰±0.09‰) to (-28.15‰±0.08‰), and the δ¹⁵N values ranged from (6.33‰±0.24‰) to 10.23‰. In autumn, the δ¹³C values of fish ranged from (-33.08‰±0.76‰) to (-28.75‰±0.68‰), and the δ¹⁵N values ranged from (6.43‰±0.76‰) to 10.13‰. Statistical analysis indicated no significant seasonal differences in δ¹³C and δ¹⁵N values between summer and autumn (p>0.05). There was no significant difference in δ¹³C values between carnivorous and omnivorous fish (p>0.05), but the δ¹⁵N values of carnivorous fish were significantly higher than those of omnivorous fish (p<0.05). The trophic level ranges of the fish samples collected in summer and autumn were 2.63-3.77 and 2.66-3.75, respectively, with no significant seasonal differences in the trophic positions of each fish species (p>0.05). The food chain lengths of the Xiaoxi'ergenqi River in summer and autumn were 3.77 and 3.75, respectively. For the six isotope quantitative indices, the values in summer were all higher than those in autumn, indicating that the trophic diversity of the fish community was higher in summer than in autumn, while the degree of trophic redundancy was lower in summer. The study also found that among the three carbon sources, sedimentary particulate organic matter (SOM) contributed the most to the entire food web in both summer and autumn, while the contributions of suspended particulate organic matter (POM) and phytoplankton were relatively lower. According to the food web characteristics of the Xiaoxi'ergenqi River predicted by the Bayesian mixing model, the benthic food chain dominated the entire river ecosystem in both summer and autumn.