As an important part of grouper culture, tomato hind (Cephalopholis sonnerati) is a rare fish species in China, which has a huge consumer market and broad prospects. Gonad samples of tomato hind from 30 to 380 days were observed using dissection and histological methods to investigate the early gonadal development, gonadal differentiation, and reproductive characteristics of tomato hind under artificial breeding. The results showed that tomato hind is a typical functional protogynous hermaphrodite species with asynchronous ovarian maturation and multiple spawning events. The oocytes can be divided into six phases, and the development of ovarian can be classified into six periods based on the largest transverse section area occupied by different phases of oocytes. Primitive ovarian cavity appeared at 45 days after hatching. The ovarian cavity was observed at 60 days after hatching, indicating the beginning of morphological differentiation of the ovaria. Oogonial began to proliferate at 150 days after hatching. The ovaries developed to stage II at 270 days after hatching, with phase II oocytes predominant and oogonial being activated to proliferate. The ovaries of tomato hind developed rapidly, reaching stage III at 310 days after hatching, and the volume of oocytes increased significantly. The ovaries reached stage IV at 380 days after hatching, when the volume of oocytes increased further and oocytes were filled with yolk. Ovaries of stage V were sampled during the breeding season. The nuclear membrane disappeared, and the yolk homogenized. After the breeding season, the ovaries reached stage VI, were dominated by phase II and III oocytes, and degeneration of oocyte were observed. In many cases, the occurrence of sex change is under social control, such as the disappearance of the dominant individual. Under artificial breeding conditions, sex reversal could occur before first sexual maturation at 380 days after hatching in tomato hind. The histology showed that most oocytes were in phases II and III in the early intersex traditional phase, while a few atretic oocytes, spermatocytes, and sperm cells could be observed. We speculated that the artificial breeding population of tomato hind was a new social group, which only had females at the beginning. Therefore, when the breeding season comes, some individuals will be induced by social factors to sex reversal. This study showed the gonad development and differentiation characteristics of tomato hind under artificial breeding, providing a reference for large-scale artificial breeding and a theoretical basis for sex control breeding.