Among vertebrates, fishes show an exceptional range of reproductive strategies, which can be roughly divided into three categories: gonochorism, sequential hermaphrodite and partheonogenesis. In general, gonads can differentiate into ovaries or testes after sex determination, and remain unchanged throughout the life cycle. In contrast, some hermaphroditic fishes can change their sex from male to female (protandrous), female to male (protogynous), or serially (bi-directional sex change) in their life cycle. Hermaphroditic fishes have various forms of sex change, which is an ideal model to investigate the sex determination and differentiation in vertebrates. Therefore, this paper reviews the recent research progress on the sex differentiation and determination of hermaphroditic fishes from the aspects of gonadal change, sex hormone secretion, neuroendocrine, sex differentiation and molecular regulation of determining genes, so as to provide a reference for further research on the sex regulation mechanism of the fish.