Recently, the strength reduction method (SRM) has gained importance in static and dynamic fields of Geotechnical Engineering. Based on this method, the dynamic safety factor of slopes and tunnels has been studied by scholars. As a complicated 3-dimensional structure, the tunnel portal section is partly influenced by the interaction between the surrounding rock, slope, and lining of the structure. Seismic stability research is now mostly conducted for qualitative descriptions. To evaluate the dynamic stability of such projects and provide references for design and construction, quantitative indexes should be developed. According to the shaking table test, a numerical model of a 3-dimensional tunnel portal section is built, in which the SRM is adopted to acquire a dynamic safety factor of this section. In this study, the plastic energy criteria (PEC) combined with the catastrophe theory (CT) is used as the failure judgment of the model in deciding the safety factor. Using the CT, the critical changing point of the reduction factor-plastic energy curve is decided. The safety factor of the model then becomes the reduction factor at this critical point. Results show that the PEC combined with the CT have a small error and exhibit the advantages of simplicity and convenience compared with other failure criterion such as displacement criterion, non-convergence criterion and the criterion of plastic zone connection. The dynamic safety factor of the model is smaller than the static safety factor, which is in accordance with the law that structures exhibit lower stability under dynamic loading. The dynamic safety factor is 1.52 for the model, indicating that the portal section in simulating is rather stable, which agrees with the shaking table test results. The results show that the PEC combined with the CT is feasible in such engineering cases, which could provide reference for similar projects.