Aiming　at　the　design　parameter　optimization　problem　of　Permanent　Magnet　Retarder　(PMR),　mathematic　model　of　PMR　electromagnetic　field　is　made.　Magnetic　field　and　eddy　current　field　in　PMR　are　numerically　solved　by　finite　element　method.　The　braking　torque　influence　of　rotor　material　conductivity,　permeability　and　the　plating　layer　is　discussed,　and　the　braking　power　formula　that　the　rotor　surface　material　has　plating　layer　is　presented.　The　simulation　results　show　that:　the　electrical　conductivity　of　the　rotor　is　about　direct　proportion　relationship　with　the　braking　torque　when　it　is　quite　small,　but　with　the　conductivity　increase,　the　braking　torque　becomes　more　and　more　saturated;　rotor　relative　permeability　has　little　effects　on　braking　torque;　the　braking　torque　of　the　high-conductivity　and　non-magnetic　rotor　material　is　far　less　than　the　soft　magnetic　materials;　plating　layer　can　increase　the　maximum　braking　torque　and　the　maximum　braking　torque　move　to　the　low-speed,　but　it　will　lead　to　faster　decline　in　braking　torque　at　high　speed.