The　conventional　method　to　evaluate　the　transition　characteristics　of　the　hard-to-soft　part　of　B-pillar　reinforcement　plate　used　in　automobile　is　to　test　the　local　hardness　by　destructive　sampling,　which　is　not　convenient　and　efficient　for　the　parts　directly,　and　is　time-consuming　and　costly.　Hence,　magnetic　Barkhausen　noise　measurement　method　is　investigated　for　nondestructive　evaluating　the　transition　characteristics　to　overcome　the　shortcomings　of　traditional　methods.　Firstly,　the　self-developed　multi-functional　micro-magnetic　instrument　is　used　to　carry　out　calibration　experiments.　Then,　the　estimation　method　of　the　position　where　the　transition　zone　begins　and　ends　and　length　of　the　transition　zone　is　proposed.　Subsequently,　a　quantitative　prediction　model　of　surface　hardness　in　transition　region　is　established.　The　results　show　that　most　of　the　feature　parameters　of　MBN　and　TMF　can　effectively　evaluate　the　transition　range　of　hard-to-soft　part.　The　estimation　error　for　the　starting　point　is　±14　mm,　and　the　relative　error　of　length　estimation　is　less　than　10%　using　parameters　of　x1,　x5　and　x17.　The　RMSE　of　linear　characterization　model　with　parameter　of　x17　is　about　12.38　HV10,　while　BP　neural　network　model　established　by　integrating　multiple　magnetic　parameters　has　a　higher　prediction　accuracy　with　a　RMSE　value　less　than　6　HV10.　Obviously,　a　micro-magnetic　evaluation　method　for　the　transition　characteristics　of　the　hard-to-soft　part　of　key　automotive　components　is　proposed,　in　which　the　relationship　between　feature　parameters　from　magnetic　Barkhausen　noise　and　hardness　is　studied　and　the　quantitative　characterization　of　surface　hardness　and　transition　range　of　hard-to-soft　part　of　B-pillar　is　realized.　The　method　has　broad　prospects　in　engineering　application.　©　2023　Editorial　Office　of　Chinese　Journal　of　Mechanical　Engineering.　All　rights　reserved.