Mutualism is the relationship that exists in two organisms of different species in which each individual benefits from the activity of the other. In this paper, some typical symbiosis examples between ascidian and cyanobacteria are introduced to explain the discovery process of their symbiotic relationship, the transmission mechanism of cyanobacteria between ascidian generations, and the possible physiological function of both sides in the symbiosis relationship. For example, cyanobacteria symbionts can both provide nutriment and participate in defense for the ascidian host by means of carbon fixation, nitrogen recycling and metabolites production. Ascidian host can provide a part of nitrogen nutrients for the cyanobacteria symbionts’ growth and protect them from ultraviolet radiation. The study of symbiotic relationship between ascidian and cyanobacteria will help us understand the process of biological evolution, and provide research foundation for further utilization and development of symbiotic relationships between marine organisms.