The invention discloses a security cloud storage system based on increment synchronization. A three-tier architecture of client-intermediate server-private cloud terminal is adopted, and data security is guaranteed by multiple security protection technology. After being blocked and encrypted, a user file is transmitted to a cloud terminal through an SSL (security socket layer) security transmission protocol; after finishing the work of user management and key management, the intermediate server terminal adopts a mutual authentication mechanism and an access control mechanism to realize the safe data share so as to completely prevent original data from leaking from the server terminal; the binding of the file and a mobile device is realized at a client, the specific file can be only opened on a specific device, and even though the file is leaked by an insider, a leaking device can be traced; moreover, the unique increment synchronization technology is realized, and the file can be synchronized by only synchronizing modified file blocks, so that the flow consumption in the file transmission process is reduced, the transmission efficiency is improved, the system usability is enhanced, and user experience is promoted.