For the general no-reparable phase-mission system, the working time of each phase is usually different. In this paper, the working time of each phase of the system is discretized to a fixed value, and the reliability probability of each time is obtained by using discrete-time Bayesian network. The discrete-time Bayesian network model of each phase is constructed respectively. The directed edges are used to deal with the phase dependence problems, and the Bayesian network model of the system is obtained. In this paper, the discrete-time Bayesian network is used to evaluate the general phase-mission system, and the feasibility of this method is verified by comparing with the universal generating function(UGF) and Monte Carlo(MC) method.