In the context of modernization and urbanization, the evolution of vernacular dwellings is affected by various complex factors, and its characteristics and rules are difficult to explore. In Tongshan Ancient City, Dongshan Couty, Fujian Province, many vernacular dwellings built in various periods since the Ming Dynasty have been preserved, and their spontaneous evolution with the residents as the main construction body is unique and diversified. The renewal and preservation of local dwellings need to be based on research and knowledge of their evolutionary patterns. Therefore, to clarify the evolution process and reveal its characteristics and mechanism, this study utilized a qualitative and descriptive research method, the case study approach, based on many investigations and research, and took Ou’s dwelling on the main street as the research object. By analyzing the architectural evolution of the typical case, this study identified the evolution characteristics of gradualness, priority, and adaptability, and explained the meaning and mechanism of influence factors, four key factors are identified as economic factors, livelihood factors, policy factors, and custom factors.