A self-decoupling magneto-rheological(MR)Damper, which is used for civil infrastructure vibration control, is designed and manufactured with 360kN maximal output force. The crucial technique for designing damper, performance test of the damper and mechanical model parameters identification are mainly studied. The performance test results of the MR damper show that the damper, including decoupling device in side-piston, has different stiffness and damping in case of different amplitude. It is feasible for the inverse control of the damper to be implemented by combining the permanent magnetic and the electro-magnetic field in main piston. The application of the isolating magnetic hoop can assure the magnetic lines to run along the theoretical designed path, and also raise the durability of the damper by avoiding the direct contact between the coils and MR fluid. The parameters of arctangent function model are identified and the theoretical results are consistent with the experimental results.