Abstract: This paper takes the digital safety level DCS emergency shutdown system which used 2-out-of-3 architecture as the research object, and establishes the reliability model of the system by the method of Markov. The average probability of failure on demand, as so called the PFD_avg, under two cases of common cause failure and non common cause failure consideration are calculated and compared. In addition, it turns out that the PFDavg changes to be bigger with the increasing of the factor of common cause failure. Thus, in order to decrease the factor of common cause failure, it is necessary to control the common cause failure by some effective measures when designing the system to improve the reliability of RTS.