With respect to the improved Auxiliary Feedwater System(ASG) of Nuclear Power Plant(NPP), this paper studies the safety evaluation method based on the Bayesian Network(BN). The first step is to set up the Reliability Block Diagram(RBD) of ASG through the Failure Mode and the Effect Analysis(FMEA). Then the BN of the ASG is built in terms of the mapping relation between RBD and BN. Based on the network reasoning, the event occurrence probability is calculated and the system reliability is analyzed. It is indicated that through the forward reasoning, the backward reasoning and the bidirectional reasoning, the occurrence probability of an arbitrary node can be calculated and more information about the reliability evaluation can be obtained. Compared with the traditional Fault Tree Analysis(FTA), this method enjoys a more powerful analysis ability and its results are more closely verified by practice. It can be applied to the reliability analysis of the safety system in NPP.