In a superconducting suspension system, the disturbance torque acting on the superconducting rotor may not be generated if the rotor is an ideal sphere and in the complete Meissner state. However, in fact there exist always spherical tolerance during manufacturing process of the sphere and the centrifugal distortion due to the high speed rotation. Therefore, the disturbance torque will be generated due to the magnetic levitation force not getting through the mass center of the rotor when the rotor is levitated in the magnetic field. Based on the physical mechanism of the superconducting-magnetic bearing, the disturbance torque and the drift error are analyzed. The disturbance torques include the main torque due to asphericity of the sphere, the second torque generated by the combination of asphericity, uncentering and assembly errors. The model of drift rate is also deduced and the drift rate is calculated by substituting the rotor parameters into the formula. This analysis provides a reference for the rotor drift testing and error compensation, and is instructive for the optimization design of the rotor structure.