By analyzing the problem of security and reliability existing in the traditional file system , this paper firstly proposes a security mechanism based on linux OS which means that the OS will be installed into a relatively independent space and separated with other software. Meantime, the phisical storage medium is only-read to make the OS un-rewritten. Users can install the software and save documents pellucidly in the user's space which is isolated with system space in phisical space. For the users, the views or the operation of the document system is as same as the traditional OS and the rock-bottom system space and user's space can not be felt. The principle of design and its practical details of space FS system are described particularly. Finally, the test data prove that the system is safe and reliable without performance loss.