Abstract: In the early days,it was thought that there were few simple microbial communities in the fish gut,but with the development of fish gut microecology,it was gradually recognized that there were a large number of diverse microbial communities in the fish gut.Complex living environments,different developmental stages,and variable food sources can lead to changes in the diversity and structure of fish gut microorganisms.The microecosystem formed by fish gut microbes and their habitat is able to maintain the host's gut health when it is in dynamic balance.Numerous studies have shown that microecosystem in homeostasis can regulate the balance of gut microbial communities and improve the composition of gut microbial communities,thus promoting fish growth and development,nutrient metabolism,immune regulation and other processes.This paper reviewed the research methods,diversity and functions of fish gut microorganisms,and summarized the effects of environmental factors such as temperature,salinity and food in water on fish gut microorganisms,in order to provide inspiration and reference for future research development of fish gut microorganisms.