In view of the poor heat dissipation of the coil and the poor sealing of the coil in the traditional coil built-in magnetorheological (MR) damper, a new type of coil external magnetorheological damper was designed. Through the magnetic rheological reduction design, device structure design of magnetic pendulum,constraint equations between the parameters of the magnetic circuit was established, which provides a theoretical basis for optimization design; and the use of parametric modeling and finite element optimization method (FEM), the optimization of structure parameters of external magnetic circuit of magnetorheological damper was carried out. The optimization and simulation results verify the rationality of the final structural parameters. The correctness of the analysis results is verified by the static damping test.