Risk assessment is one of important technologies to ensure the safety of the long-distance oil/gas pipeline, and calculating the failure probability is the important part of the risk assessment, then the veracity of the failure probability impact the rationality and applicability of the result of the risk assessment. In this article, statistical method and analytic method which could be used to compute failure probability is introduced, and fault tree analysis (FTA) of statistical method is fit for ensuring the failure probability when there is no calculating model and history data. By the D-S theory, unilateral of for only one expert could be avoided, making the judge of the many experts more true and firstly put forward integration evidence combination rule. By this way it is not only make the calculating model of FTA to compute the failure probability perfect, but also bring forward the thought to deal with uncertainty information such as ensure the failure probability by judge of the expert when the failure model is scarcity.