This paper proposed an information security Bayesian network (IBN) model to evaluate the system information security. First, an information security hierarchy model is put forward with the analytical hierarchy process based on the characteristics of equipment development enterprises information systems and information security evaluation criteria. Then, according to the definition of Bayesian network, the IBN is built by transforming the information security hierarchy model into Bayesian network with structure and probability distributions. Finally, a case study is implemented to demonstrate the proposed evaluation method. The results show that the IBN model provides a convenient framework to evaluate the system information security.