With　motor　vehicle　developing　toward　heavy　and　large-size,　its　braking　safety　issue　becomes　increasingly　prominent.　Additionally　installing　auxiliary　braking　device　is　one　of　the　important　ways　to　achieve　the　safety　braking　for　heavy-duty　vehicles.　Magnetorheological　(MR)　brake　has　advantageous　features　of　stable　braking　torque,　low　noise　and　small　volume　and　mass　etc.,　so　is　able　to　remedy　the　inadequacies　of　traditional　auxiliary　braking　devices,　e.g.　the　poor　performance　at　low　speeds.　A　new　idea　of　combined　retarding　braking　by　utilizing　both　MR　and　eddy-current　effects　is　put　forward,　the　working　principle　of　novel　MR　retarder　is　analyzed,　the　double-disk　symmetric　structure　of　novel　retarder　is　designed　and　developed,　the　mathematical　model　for　its　braking　torque　is　established　and　the　effects　of　different　parameters　on　braking　performance　are　theoretically　analyzed.　According　to　the　working　features　of　high-power　braking　in　heavy-duty　vehicles,　a　testing　platform　for　braking　performance　is　constructed,　and　a　test　on　braking　torque　characteristics　is　conducted.　The　theoretical　analysis　and　test　results　show　that　the　novel　MR　retarder　has　the　features　of　stable　braking　performance　at　low　speed　and　large　braking　torque　at　high　speed,　with　its　braking　torque　characteristics　meeting　the　braking　requirements　of　heavy-duty　vehicles.　©　2019,　Society　of　Automotive　Engineers　of　China.　All　right　reserved.