Abstract: The performance of geosynthetic-encased stone columns in composite foundation is investigated through the finite element numerical simulation that is verified by the published experimental results from model tests, and then the effects of length of geosynthetics on the failure modes and the ultimate bearing capacities of single column and group columns are analyzed. The research results show when the stiffness of the geosynthetics is low, the length of the geosynthetics will not affect the shear failure modes, and the improvement of the ultimate bearing capacity is limited. With the increase of the stiffness of the geosynthetics, the shear failure of the stone column occurs in the area below the geosynthetics-encased body, and the ultimate bearing capacity of the foundation increases linearly with the increase of the length of the geosynthetics to the deeper soil layer. The effects of the length of the geosynthetics on the failure modes of stone column at different positions of composite foundation with pile groups are different. Compared with the side columns, the center columns have shear failure at the deeper position of the stone body, and the geosynthetics needs to reach a deeper length to exert the restraint effects.