Abstract: The factor of safety (F_S) is a very important design index for deep excavation in soft clay. The existing methods for F_S mainly include the conventional methods and finite element method (FEM) based on the reduced shear strength technique. In this study, the conventional methods and reduced shear strength FEM are used for F_S evaluation of a failed deep excavation in soft clay. By comparing the values of F_S of the excavation using different soil strength parameters and methods, the applicability of the analysis methods and soil strength parameters is evaluated. The research results indicate that the values of F_S of the excavation calculated by the conventional methods and the reduced shear strength FEM using the total stress indexes, i.e., s_u, =0, are very close to 1.0 and those from Terzaghi’s method, while the values of F_S of the excavation are much greater than 1.0 using the soil strength parameters c and from triaxial tests. When the reduced shear strength FEM is applied for F_S evaluation, the values of F_S obtained from the convergence criterion method are much greater than those from the intersection method. The values of F_S of the excavations using the intersection method and Terzaghi’s method are both very close to 1.0, which is consistent with the limited equilibrium state of the excavations at failure.