The electromagnetic damper studied in this paper belongs to a special kind of coreless generator, which is primarily used for the energy absorption and dissipation in space docking mechanism buffer system. The parameters of magnetic circuit tructure which determines the air-gap flux density are important factors in affecting damping torque. Three-dimensional models are built based on MagNet of Infolytic corporation in this paper. The effects of structure parameters of magnetic circuit such as magnet material, magnetic pole-pairs, rotor thickness on the torque-speed characteristic are researched. The analysis results show that the damping torque is proportional to the square of the amplitude of the fundamental component of the air gap flux density, rotor thickness respectively, the relationship of the torque-speed ratio with the magnetic pole is neither directly proportional nor inversely proportional.