The damage grade assessment of the sap gateway is an important basis for the rush-repair scheme making. The traditional method can only give the qualitative result, but can't provide quantitative result, thus a greater error is caused in assessment result. First, this paper gives the partition criterion of the damage grade, then analyzes the factor that influences the damage degree of the sap gateway and determines the attribute and values. Furthermore, the Bayesian network for assessing the damage grade of the sap gateway is established, the network parameters are given by utilizing the experiences of experts, and the united probability distribution formula of the damage degree is deduced. Finally, an example is given, which is solved by reasoning platform GeNIe. The conclusion is drawn that the Bayesian network model can introduce the random factors influencing the damage degree, and by well combining the experiences of experts with the damage data, the probability relationship of the variables is given. The assessment result is consistent with the numerical simulation result.