Abstract: The onboard computer(OBC) that controls a manned spacecraft must be extremely reliable．Such a computer system generally has the ability of Byzantine fault resilience．This paper investigates the reliability of the quadmodular redundancy Byzantine fault tolerant computer. By using a continuoustime discretestate Markov model, the occupancy probabilities of the system are calculated. Several probabilities pertinent to reliability are recorded. The proposed approach shows that which redundancy scheme we adopt should depend on the relationship between Punsafe and Pshutdown, and provides reference for the design of the quadmodular redundancy Byzantine fault tolerant computer.