With the increasingly prominent problem of global climate change, the impact of ocean acidification on Marine ecosystem has been widely concerned. Sargassum thunbergii, as the dominant species in the intertidal zone, is one of the important components of the Marine ecosystem and has important ecological and economic value. Epiphytic bacteria interact with algae and play an important role in the growth and development of algae. Therefore, it is of great ecological significance to investigate the effect of acidification on the community structure of the epiphytic bacteria of the algae. Algae and bacteria in the ocean are inseparable and have intricate interactions, which together constitute an important regulator of the structure and function of the Marine ecosystem. The increase of CO2 concentration and decrease of pH in Marine ecosystems have significant effects on epiphytic bacteria, especially the interaction between macroalgae and epiphytic bacteria. Ocean acidification may have a directed selection effect on different adaptation degrees of Marine microbial populations, thereby reshaping microbial communities and influencing the composition and diversity of epiphytic bacteria in macroalgae.Maintaining higher biodiversity and healthier microbial-host interactions can go a long way toward mitigating the negative effects of acidification on other highly vulnerable species, and further understanding of the host-bacterial relationship has theoretical and practical value. At present, there are few studies on the effects of ocean acidification on the epiphytic bacterial community of algae, especially on the sex differences in the composition of epiphytic bacterial community of dioecious macroalgae under ocean acidification. In the context of continuous ocean acidification in the future, it is of great ecological significance to investigate the changes in the community structure of the epiphytic bacteria of rat-tail algae, which is helpful to evaluate the impact of ocean acidification on the Marine ecosystem in the future.