Abstract: Collichthys lucidus is an important small economic fish, and in order to understand its nutritional structure and dietary characteristics, we collected samples from the waters of the Pearl River Estuary in 2021 and 2022, and studied them from the perspectives of trophic niche, trophic structure index and feeding habit by on stable isotope technology. The results indicate that the δ¹³C and δ¹⁵N values of C. lucidus were (−18.74±1.41)‰ and (12.35±0.57)‰, respectively, with a trophic level of (3.25±0.17)‰. The δ¹³C value gradually increased when the body length was less than 100.00 mm, then decreased with the increase of body length. The δ¹⁵N value increased with the increase of body length when the body length was less than 110.00 mm, and the inflection point occurred at body length of 110.00–120.00 mm. The δ¹⁵N value was the lowest, and then increased with the increase of body length. When the body length was less than 100.00 mm and more than 120.00 mm, the trophic diversity was high, and the trophic niche width was large. When the body length was 100.00–120.00 mm, the redundancy and evenness were high, and the trophic niche distribution was uniform in the community, and the individuals with similar trophic characteristics accounted for the majority. Bayesian mixing model tracing reveals that the highest food contribution proportion for C. lucidus was from zooplankton. The research provides a theoretical basis for the resource conservation and management of C. lucidus.