Abstract: The pile-soil interaction in an expansive soil foundation is complicated, and the influence of the inundation swelling deformation on the torsional bearing characteristics of the pile foundation remains unknown. In this paper, the displacement governing equations for the pile shaft were proposed considering the load transfer method. The distribution of the vertical resistance of the shaft was calculated by the finite difference method considering the influence of the swelling deformation of the expansive soil on the shaft resistance and the boundary condition of the pile tip. A new nonlinear analytical method for the torsion of a single pile due to the inundation swelling deformation was proposed incorporating the boundary element method. The distribution law of the circumferential frictional resistance of piles was revealed and the effectiveness of the proposed method was verified by model tests. It was shown that the ground heave induced by the swelling of the expansive soil foundation would significantly reduce the torsional bearing capacity of the single pile. In addition, the safety is threatened because the conventional calculation method overestimates the ultimate torque of the pile and the loading stiffness of the pile-soil system. The torque of a single pile approximately decreases linearly along the pile shaft prior to the inundation swelling deformation. However, the torque of the pile shaft is close to that of the pile top and the torque of the pile shaft decreases rapidly in the “unslip section”. The length of the interface “slap section” increases and torsional force capacity of the single pile decreases with the increase of the swelling ratio.