In this paper, the risk of system failure was obtained through Bayesian probabilistic risk assessment method for aircraft power system. Bayesian models of (Ga-Exp) and (D-M) as to common cause failure group were established in order to obtain the Bayesian estimation of the failure rates of the five types of components and the common cause failure ratio of each order. According to the Alpha factor model, the failure rate of the components in the common cause failure group was estimated. Finally, the Bayesian risk probability of system failure was studied. The sensitivity of component failure to system failure risk was defined as component importance index. By a practical example, with the cumulative flight time, the risk of system failure, the importance of common cause failure group components and the five types of components were compared in the two cases of whether considering common cause failure. The validity of the model and the necessity of considering common cause failure factors were verified so as to provide the basis for maintaining the reliability of aircraft.