Chlorurus sordidus is a protogynous hermaphroditic coral reef fish that exhibits the phenomenon of spawning agglomeration and is prone to overfishing. C. sordidus is one of the most abundant reef fish in coral reef ecosystems and is widespread on coral reefs throughout the Indo-central Pacific. The herbivorous characteristics of C. sordidus give it the ability to descend and control algae in coral reefs, thereby promoting coral attachment and growth, which has a very important ecological value. Samples were collected from the Yongle Atoll and Meiji Reef by dive fishing in July 2020 and April 2021. Samples were brought back to the laboratory for dissection and analysis after quick freezing. This study determined the biological characteristics of C. sordidus, such as standard length and body weight characteristics, sex ratio, 50% sex reversal standard length, gonadosomatic index, egg size characteristics, and fecundity, with the goal of better protecting and managing this coral reef fish. The results showed that the average standard length and body weight of the Yongle Atoll and Meiji Reef males were significantly larger than females (P＜0.05), but there was no significant difference in the average standard length and body weight of females and the average standard length and body weight of males between the two places (P＞0.05). There was no significant difference in the length-weight relationship between the Yongle Atoll and Meiji Reef, with the relationship being W=9.67×10^–5×L^2.809 and W=7.27×10–⁵×L^2.877, respectively. The male-to-female ratios at the Yongle Atoll and Meiji Reef were significantly different, and the sex ratios were 1.33 1 and 6.33 1, ∶ ∶ respectively, and the male ratio was significantly higher in the Yongle Atoll. The 50% sex reversal standard lengths at the Yongle Atoll and Meiji Reef were 151.60 mm and 174.37 mm, respectively, and the sex reversal standard length of the Meiji Reef was larger than that of the Yongle Atoll, indicating that the female fish in the Meiji Reef did not revert to male until they had grown to a larger size. There was no significant difference in the gonadosomatic index of female gonads at stages III and IV between the two places, which were 1.74 and 2.87 at the Yongle Atoll and 1.86 and 2.77 in the Mischief Reef, respectively. The distribution of egg diameter in both places was continuous, which confirmed that C. sordidus was a typical batch breeding fish. The fecundity and relative fecundity of standard length and body weight of the Meiji Reef were significantly higher than those of the Yongle Atoll. The fecundity and relative fecundity of standard length and body weight at the Meiji Reef were 82941 eggs, 545.59 eggs/mm, and 693.52 eggs/g, respectively, and at the Yongle Atoll were 53540 eggs, 328.20 eggs/mm, and 442.80 eggs/g, respectively. The fecundity of Meiji Reef had no significant relationship with body length and body mass, but there was a significant positive correlation in the Yongle Atoll. The results indicates that fishing pressure had obvious plastic changes in the basic biological characteristics of C. sordidus, especially the sex ratio and fecundity, both reached significant differences. The results of this study provide a basic reference for fish plasticity research and basic data support for the ecological restoration of islands and reefs.