Abstract: The calculation of the mission reliability of a fault-tolerant standby storage system is discussed.Whether the fault detector,switch and the standby component are reliable or not affect the reliability of the fault-tolerant system directly.In order to evaluate its mission reliability correctly,the detector and switch which have a certain false detection probability and false switching probability are considered in this paper.To our knowledge,Markov model is the main tool to deal with this problem which is seldom discussed in China.Here Markov theory is applied to determine the system states and their corresponding state transfer matrix,by which the problem of calculating fault-tolerant reliability can be solved.Moreover,the calculation models of some relative error probabilities are discussed.