Abstract: The hooksnout carp (Opsariichthys bidens) exhibits pronounced sexual dimorphism in growth, with males attaining a growth rate more than 1.5 times that of females. In addition, males develop distinctive nuptial color stripes, which enhance their visual recognition and ornamental value. As such, the production of all-male populations holds significant potential for improving aquaculture efficiency. To elucidate the biological mechanisms underlying sex differentiation and to establish a technological system for generating all-male offspring, this study first systematically characterized the sexual dimorphism of the species through observation of natural populations. Histological analysis was then employed to track the continuous developmental process from hatching through the completion of sex differentiation, identifying the critical period for sex differentiation as occurring between 35 and 100 days post-hatching (dph). Accordingly, exogenous sex reversal was induced via dietary administration of 100 mg/kg 17β-estradiol (E₂) from 25 to 70 dph. Following treatment, gonadal phenotypes comprised ovaries (86.0%), intersex gonads (11.1%), and testes (2.9%), confirming the successful production of fertile XY pseudo-females. Subsequent crosses between these XY pseudo-females and normal XY males successfully generated YY super-males. This study systematically defines the critical period and histological characteristics of sex differentiation in hooksnout carp, demonstrates the efficacy of dietary E₂ administration in inducing sex reversal in XY individuals, and establishes a foundational genetic breeding pathway from XY pseudo-females to YY super-males. These findings provide both a theoretical framework and a technical basis for the development of a novel all-male germplasm in hooksnout carp.