Based on the deformation characteristics of the composite foundation reinforced with encased stone columns under vertical loads, a new calculation formula to estimate the pile-soil stress ratio of geosynthetic-encased stone columns was proposed with consideration of pile-geosynthetic-soil interactions. In the derivation process, initial stresses in soil and a column were taken into account, the column was considered as an elastic-plastic material with constant dilatancy angle and satisfying both the Mohr-Coulomb yield criterion and non-associated flow rule, and the soil and geosynthetic encasement were considered as linear-elastic materials. The proposed formula was validated by comparison with the results of elasto-plastic limit analysis method. Finally, a parametric study was conducted to investigate various parameter effects of encasement stiffness, deformation modulus of soil, and replacement ratio on the behavior of the pile-soil stress ratio. The parametric study shows that the pile-soil stress ratio of the composite foundation with the encased stone column increases with the increase of encasement stiffness, replacement ratio of composite foundation and friction angle of the column, but decreases with the increase of deformation modulus of soil and dilation angle of column. Moreover, among all these parameters, the encasement stiffness hasd a significant influence on the pile-soil stress ratio.