Abstract: Overfishing has led to small fish species such as the Leiognathidae family gradually becoming the absolutely dominant species in the inshore waters of the South China Sea. During the 2024 fishery resources survey in Daya Bay, three small-sized leiognathid species (Equulites berbis, Nuchequula nuchalis and Leiognathus brevirostris) formed a high-density coexistence pattern. Analyzing their coexistence mechanisms will provide foundational support for core theories in ecology. This study used carbon and nitrogen stable isotope techniques to explore their coexistence mechanisms. The results indicate that the trophic levels of all three Leiognathidae species increased significantly with body length. The trophic level of E. berbis was 3.20, representing an omnivorous and carnivorous diet, and was significantly lower than those of the other two species (p<0.05), indicating that it realized trophic niche differentiation by feeding on prey at lower trophic levels. Approximately 90.73% of the trophic niche area of N. nuchalis was nested within that of L. brevirostris. However, their δ¹³C values differed significantly (p<0.05), suggesting fundamental divergence in food source composition despite intense trophic competition. This study provides isotopic niche evidence for the biodiversity maintenance mechanisms of leiognathid fishes in the South China Sea, offering a scientific foundation for their sustainable resource management.