Research on archival characteristics of social media platforms contributes to more mature and feasible archival management of social media information,and also promotes the digital transformation of archival management theory and methods. The paper takes the method of theory building to explain the connotation and characteristics of social media platform as the archival repository from the formation of social media archives, the archive essence and new features. This paper demonstrates the idea that social media platforms can be regarded as archives,and puts forward that the formed social media archives derive from social media information repository based on active information activities,the construction of archival repository is realized due to its archival value and increasingly common archival management behavior,and the archival repository also presents new features in terms of time and space,subject,relevance and value,thus becoming an important field of the digital transformation of archives management.