Abstract: Reservoir plays important roles in realizing the sustainable development of economy and society. It can provide water resource to human beings for living, industrial and agricultural usage, and for flood control, power generation and regional ecological service as well. The sustainable utilization of the reservoir water resource is the prerequisite for fishery industry development. However, following with the impacts of increasing human activities, pollutant input and water eutrophication, the ecological environment gets deteriorated overtime, which was revealed by water balance disorders, such as sharp decrease of biodiversity and others. In order to better understand the trophic structure characteristics of reservoir ecosystem in China, Songlin, a typical small reservoir in Southeast Sichuan province, was selected for the stable isotope analysis. The basic carbon sources of different consumers were analysed based on the Bayesian mix-model (SIAR), and six nutrition structure quantitative indicators in δ¹³C—δ¹⁵N isotopic niches were calculated as well. Results showed a eutrophication state of this small reservoir, and the δ¹³C and δ¹⁵N values of primary producers such as POM (mainly the planktonic algae), periphyton, Alternanthera philoxeroides and Polygonum hydropiper were within the scopes of –29.20‰—18.81‰ and 4.01‰—12.73‰, respectively. POM and periphyton were found as the main carbon sources for most consumers. The food web trophic level length of Songlin reservoir was calculated at Level 3, with omnivorous fishes as superior group and existed the nutrition redundancy phenomenon, which suggested a relatively stable fish community structure in Songlin reservoir. Additionally, niche overlaps and competitions were found between invasive species (such as Pomacea lineata and Oreochromis sp) and native species (such as Bellamya aeruginosa and Carassius auratus, etc). Taking together, we suggested to enhance the management of aquatic organism resources, and powerful strategies must be taken to prevent and control the invasive alien pest for better protection on the biodiversity of Sonlin reservoir.