A secure decentralized storage system provides scalable security by addressing the performance bottleneck of the security manager and the complexity issue of security administration in large-scale storage systems. The storage system includes: an application client for accessing a file system using a plurality of storage devices and transmitting a command to a storage device; a storage device for storing data and access control entries associated to the data, analyzing the command from the client and performing corresponding operations of the command; a metadata server for storing and managing metadata, such as location and length information of data and system configuration; and a security manager for storing and managing global access control entries and policies of the system and performing the access policy and privilege control according to the global access control entries and policies, such as changing the priority and inheritance rule of access control entries, adding and deleting the access control entries.