A　new　permanent　magnet　retarder　(PMR),　which　based　on　the　principle　of　eddy-current　braking,　is　proposed　as　an　auxiliary　braking　apparatus　for　heavy　vehicles.　Its　braking　torque　can　be　stepless　adjusted　by　the　adjustment　mechanism.　A　multi-physics　field　coupling　model　of　the　retarder　was　established,　which　includes　electromagnetic　field,　thermal　field,　and　fluid　field.　The　distributions　of　the　three　fields　were　calculated　using　the　finite-element　method.　To　predict　the　demagnetization　of　the　permanent　magnet,　a　new　analysis　method　of　the　permanent　magnet　irreversible　demagnetization　is　proposed,　which　uses　the　dynamic　air-gap　flux　density　simulated　under　various　temperature　B-H　curves　and　the　relationship　between　the　braking　torque,　as　well　as　the　square　of　air-gap　flux　density.　The　braking　torque　and　temperature　rise　of　the　PMR　were　tested　on　the　bench.　The　bench　test　results　show　that　the　braking　torque　calculated　by　the　multi-physics　coupling　method　agrees　better　with　the　experimental　data,　and　the　high-temperature　irreversible　demagnetizations　of　the　permanent　magnet　occurred　by　the　enormous　heat　generated　during　continuous　braking.　The　vehicle　test　results　show　that　the　braking　distance　can　be　shortened　by　34%　after　using　the　PMR,　and　the　braking　deceleration　is　increased　by　1.05　m/s(2).　The　proposed　PMR　can　meet　the　requirements　of　auxiliary　braking　for　heavy　vehicles.