Siganus canaliculatus and Halichoeres nigrescens are the dominant fish in Wanshan Islands. The trophic niche and the competition relationship between the populations were revealed by the carbon and nitrogen stable isotope technology analysis to explore the reasons of the two fish species became the dominant species in Wanshan Islands. The samples in this sea area were collected by three-layer gillnet in the spring (March) and autumn (September) of 2021. Then they were measured by Elementar isoprime visION stable isotope mass spectrometer, and the trophic niche and structure were analyzed. The results showed that the δ¹⁵N values ranged from 9.15‰ to 14.09‰ of S. canaliculatus and 10.34‰ to 14.60‰ of H. nigrescens. The δ¹³C values ranged from -19.87‰ to -14.54‰ of S. canaliculatus and -18.16‰ to -15.60‰ of H. nigrescens. The trophic level of S. canaliculatus and H. nigrescens was 2.97 and 3.23, respectively. The analysis of δ¹³C-δ¹⁵N trophic niche structure showed that there was a resource competition between the two populations. The trophic niche breadth and trophic diversity of S. canaliculatus were higher than those of H. nigrescens, indicating that S. canaliculatus had more food sources and stronger ability to compete and use resources. Pearson correlation analysis showed that there was no significant correlation between body length and δ¹³C of the two populations, but there was a significant correlation between body length and δ¹⁵N. There was a significant positive correlation of S. canaliculatus and a very significant negative correlation of H. nigrescens. The results indicated that the feeding patterns of the two communities changed with ontogeny, and the S. canaliculatus tended to be high-trophic diet organisms, while the H. nigrescens tended to be low-trophic diet organisms. There was a sharp increase in the trophic niche width of S. canaliculatus and H. nigrescens in the range of 90-100 mm and 100-110 mm, respectively, which was just near the length of their first sexual maturity or sexual reversal, suggesting that the feeding habits of the two species changed during the period of gonadal development and maturity. This study revealed the trophic niche characteristics of the two dominant species in Wanshan Islands, which can provide important data reference for the study of the trophic structure and material recycling process of the whole food web in Wanshan Islands and support the conservation and rational utilization of the population, and the assessment of ecological capacity.