The finite difference method is used to numerically simulate the complete bearing and failure process of a single stone column under vertical loading in soft soil. The plastic hardening constitutive model considering volumetric strain interception is used to simulate the stone column，which can well reflect the nonlinear dilatancy mechanical behavior of the stone column and the resulting column-soil interaction. The bulging deformation of the stone column，the evolution of soil stress acting on the stone column，the failure modes and the corresponding typical p-s curves are analyzed. The overall shear failure mode assumed by the traditional formula is only applicable to soils with high stiffness，while the local shear failure mode is more common in soft soils. Therefore，based on the theory of circular hole expansion in elastoplastic media，the calculation formula of ultimate bearing capacity of single stone column considering the column bulging deformation and the soil stiffness is derived. The formula is verified by the numerical simulation solution. The bearing mechanism and failure mode of the single stone column in soft soil are systematically studied，which provides a theoretical foundation for further study on the bearing mechanism of composite ground reinforced with stone columns.