Abstract: The upper bound solution based on the classical Prandtl mechanism usually overestimates the safety factor of basal stability of foundation pits considering the effects of inserted depth of wall penetration. The solution is not commonly used in practice. At first, the effects of inserted depth of wall penetration are analyzed by means of the shear strength reduction finite element method (SSRFEM) and multi-block upper bound method. Then, an improved analytical solution based on a modified Prandtl mechanism is given, which obeys the upper bound theorem of plastic limit analysis. Verification of the proposed formula is finally carried out by comparisons between the numerical results from SSRFEM and the multi-block upper bound method. The formula is applicable in practice when a hard stratum is present well below the base of foundation pits, which is a common phenomenon in geotechnical engineering.