Abstract: Predation is a fundamental ecological process that structures fish communities. Comprehension of diet composition and trophic niche characteristics is imperative for elucidating interspecific trophic relationships, coexistence mechanisms, and optimizing food web structure and function. In this study, a comprehensive set of samples was collected from Erhai Lake, encompassing fish, particulate organic matter (POM), sediment organic material (SOM), plankton, and benthic organisms. The methods such as Quantitative Fatty Acid Signature Analysis (QFASA) was employed to characterize diet composition and trophic niche structure of dominant fish community. QFASA results indicated that planktonic fishes (Hemiculter leucisculus, Neosalanx taihuensis, and Hypomesus nipponensis) primarily relied on zooplankton, whereas bottom-dwelling fish species (Cyprinus carpio, Cyprinus carpio chilia, and Rhinogobius cliffordpopei) consumed Macrobrachium nipponense, Corbicula largillierti, and zooplankton. It is noteworthy that Pelteobagrus fulvidraco demonstrated piscivorous tendencies, with small fish constituting 54.42% of its diet. Trophic niche breadth (Standard Ellipse Area corrected, SEAc) exhibited significant variation among species, ranging from 0.40 to 9.30. Specifically, Carassius auratus exhibited the highest SEAc value of 9.30, followed by C. chilia (SEAc=7.31) and C. carpio (SEAc=6.43), reflecting their capacity for generalized resource utilization. Conversely, N. taihuensis (SEAc=1.00), H. leucisculus (SEAc=0.58), and H. nipponensis (SEAc=0.40) exhibited constrained niche breadth, reflecting specialized trophic strategies. The extent of trophic niche overlap ranged from 0 to 77.3%. The highest overlap was observed between P. fulvidraco and C. chilia (77.3%), though QFASA revealed P. fulvidraco avoided direct competition through predation on small fish. High overlap also occurred between H. leucisculus and A. nobilis (72.5%), as well as C. auratus and C. chilia (50.2%), likely driving interspecific competition due to shared habitats and diets. These findings suggest that zooplanktivorous fish should be prioritized in community regulation, while emphasizing the conservation of population resources for indigenous species such as C. chilia. Integrating QFASA with trophic niche analysis effectively elucidates the trophic structure of fish community and reveals potential interspecific competition, providing theoretical insights for fish community regulation and food web optimization of Erhai Lake.