In the development process of B and T lymphocytes, variable region (V), the diversity region (J) and joining region (D) of BCR and TCR loci (Tcrδ) are target of V(D)J recombination, which is completed under the recombination activating protein (RAG-1 and RAG-2). This rearrangement mechanism endows the body with a huge library of antibodies to resist the multiple pathogenic microorganisms in nature. RAGs structure is divided into a core and a non-core area, while the core area exerts an enzyme cutting function, and the non-core region, such as nonamer and zinc finger, plays a regulation role in V(D)J recombination. The expression of RAGs is regulated at the gene and protein levels with stringent temporal and spatial condition, in order to ensure the normal development of B/T cells. More and more reports of RAGs structure and function are found in freshwater and marine fish. According to the characteristics of RAGs and the stability in the evolution process, RAGs have been considered as wonderful indictors to research on fish immune system and system evolution. Based on the study of red-spotted grouper immune system, the development process of the immune system were discussed when RAGs were used as a marker, which would provide the basis for artificial propagation and fish vaccine.