The reliability of security and stability control device is important to the operation of electric power grid. Taking SCS-500E which is one kind of security and stability control device for example, the components and their logical relationship in the hardware system of the device are analyzed. Based on the analysis, the fault tree model and the Markov state space model are built by fault tree method and Markov state space methodology. The paper believes that the device has 4 operation states which are normal operation, hidden mal-operation, hidden anti-operation, and outage. The probabilities of the 4 operation states are got by fault tree model and the Markov state space model. Other reliability indexes of the hardware system of the device which are availability, hidden failure probability, and probability of outage caused by hidden failure are also delimited. It is proved that the results worked by the method proposed in the paper are matched with the actual facts and the method is correct.