Abstract: Gnathodentex aurolineatus is one of the dominant species among coral-reef fishes in the South China Sea. To investigate the growth patterns and population characteristics of G. aureolineatus in the Qilian Islands, and to accumulate fundamental data for a deeper understanding of the population biology of coral reef fishes in the South China Sea, we collected 522 individuals of G. aurolineatus from the waters near the Qilianyu Islands in the Xisha Islands of the South China Sea from September 2020 to September 2021, and investigated some of their biological characteristics (Including body mass, standard length, sexual maturity, sex ratio, feeding intensity, and feeding habit). The results show that the body mass of G. aurolineatus ranged from 26.64 to 184.08 g, with an average of 94.02 g, and the standard length ranged from 101 to 187 mm, with an average of 150 mm. There were significant differences in the standard length distribution and body mass distribution between the two years (p<0.01). The growth parameter b of the standard length-body mass relationship was 2.84. Gonadal maturity was mainly at stage II, and the first sexual maturity standard length of females was 153.91 mm. In September 2020, the empty stomach rate was very high (40.35%), while in September 2021, the feeding levels were mainly at grades 2 and 3 (54.47% in total). G. aurolineatus primarily fed on fishes, gastropods and crustaceans, with relative importance index percentages (%IRI) of 38.29%, 24.85%, and 17.73%, respectively. It was an invertebrate feeder and piscivore species with a trophic level of 3.44. The proportion of different prey categories varied significantly among different standard length groups, with the proportion of larger prey increasing and that of smaller prey decreasing as standard length increased.