Online community has become an important phenomenon in current Chinese society, which has multi-dimensional effects on economic, political and social fields. However，the formation mechanism, the developmental dynamics and the evolution trend of online community are still not very clear both theoretically and empirically. Based on the case study of Diba, the Baidu largest social media online group, this study found: the coupling effect shaped by a stable interactive mode from media technology and the topic/event in specific social structure and context is the crucial factor of online community formation and continuation; the internal dynamic mechanism of online community development is the construction of group identity and norms, especially through the institutionalization process of community subculture; the evolution of online community is driven by two forces: the community inner structure differentiation caused by group member expansion, which result in the weakness or failure of the original group identity and norms, and the combustion effect result from the unintended consequence of special events and online collective action.