For privilege escalation problem in operating system, a user permission isolation model based on Finite State Machine (FSM) was proposed in this paper, which depicted the users' permissions as a FSM. A user's permission was mapped to a FSM, which was able to distinguish the legality of user's operation sequence. Besides, the model proved that it easily leaded to permission escalation at the shared permission points. Ultimately, through FSM, the model achieves effective identification and judgment of user permissions isolation.