Abstract: To solve the problem of subjectivity and inaccurate result in the risk assessment, this paper presents a method which is applied in Yu Liao Tunnel based on Fault Tree Analysis(FTA) and Bayesian Network（BN. This risk probability assessment method includes three methods, such as the probability intervals, the weight and confidence index and confidence interval. First, FTA was used to construct the BN, and the dependent relationships between the factors in the cases were used to calculate the Conditional Probability Table (CPT) of every node. Second, the professor’s investigations were carried out to get the survey data which is based on the small probability interval hierarchies and the weight and confidence index method. The survey data gained from professor j about factor will be presented by . All the survey data will be used to estate the sample space and the statistic, introducing the confidence interval to get the probable range of factor . After finished all the data processing, the probable range and CPT will be used to calculate the risk, it also can be used to diagnosis the accident.