Access control is the mechanism to protect the operating system. At present, most systems lack proper access control mechanism. The implementation of access control mechanisms of the existing security system depends on the specific operating system platform, thus lacking portability. This paper sums up the differences between different platforms and the implementation methods of a variety of secureity systems. It abstracts the platform-dependence of access control mechanism, and designs a platform-independent and flexible access control framework PIFAC. This PIFAC has been implemented in Windows, Linux and Rtems.