Abstract: In the excavation of bedding rock slope with multi-layer soft interlayer, the failure of slope instability is usually occurred in a limited range, which has been named as "the critical length of instable failure". This length has a decisive impact on the evaluation of slope stability and the design of retaining engineering. However, there is still not a mature solution on how to determine the critical length, that often be determined by the experience. Based on the detailed investigation and study of bedding slope at K3+115~K3+215 section of the Huaikou Town west loop line, the deformation failure process and range of the multi-layer soft interlayer in bedding slope has been analyzed by using the numerical simulation. Compared with the existing deformation characteristics, it is proved that the numerical simulation is feasible to analyze the deformation and failure process and the critical length of instable failure. The critical length determination of instable failure provides a basis for the subsequent stability calculation of slope and the design of retaining engineering, which achieves the expected purpose. Therefore, the analysis method in this paper has reference value for the bedding slope with similar characteristics.