Abstract: At present, the analysis method for basal stability is mostly based on the consolidated undrained shear strength index. The method is unreasonable in theory for the soft clay and does not consider the strength of the soil above the lowest support. To solve the above problems, the traditional analysis method for basal stability is improved. The consolidated undrained shear strength index is converted into undrained shear strength, and the strength of the soil above the lowest support is considered. Furthermore, the circular arc mechanism of the upper bound limit analysis based on the undrained shear strength is proposed. Through the calculations of engineering cases, the factors of safety obtained by the traditional circular sliding method is generally high, while the results obtained by the proposed upper-bound solution are more reasonable.