The invention discloses a cross-platform-unified-management-supported mandatory access controlling system. The system comprises a security management center; the security management center manages servers with different operating systems and data processing terminals which are connected with the servers, and the security management center also comprises mandatory access controlling modules, which connect the servers and the data processing terminals, in an application layer; and the mandatory access controlling modules comprise a special mandatory access controlling module, a general mandatory access controlling module, a strategy module and an audit module, wherein the general mandatory access controlling module is used for controlling mandatory access and checking strategy conformance. The invention also discloses a mandatory access controlling method based on the system, an access request is checked by the mandatory access controlling module based on a system mandatory access controlling strategy, a behavior which does not accord with the system mandatory access controlling strategy is checked, a behavior which accords with the mandatory access controlling strategy is regulated, the operation of a process on a file is controlled, the reliability of system security is improved, the confidentiality and integrity of an information system can be protected and prevented from being destroyed, and the flexibility of system security control is enhanced.