The　hard　spot　detection　is　one　of　the　key　contents　during　quality　evaluation　of　high-end　steel　plate　products.　In　this　study,　a　domestically　developed　instrument　based　on　the　principle　of　incremental　eddy　current　method　is　presented　for　detecting　hard　spots　in　steel　plates.　The　function　and　performance　of　the　developed　instrument　are　experimentally　examined.　First,　a　sensor　of　incremental　eddy　current　and　its　detection　circuit　are　developed　by　using　AC-bridge,　phase　rotation　and　demodulation　techniques.　As　a　result,　a　specialized　instrumentation　of　incremental　eddy　current　is　integrated.　Secondly,　several　groups　of　experimental　testing　are　performed　on　steel　plate　of　Cr12MoV　and　pipeline　steel　of　X70　with　the　developed　instrument.　The　obtained　results　show　that　the　spatial　resolution　of　the　sensor　can　reach　around　10　mm.　The　peak-to-peak　value　(Up)　of　the　demodulated　waveform　of　the　incremental　eddy　current　signal　is　used　as　feature　parameter,　which　demonstrates　approximately　linear　decreasing　trend　as　the　increase　of　the　surface　hardness.　The　value　of　Up　is　decreased　by　at　least　about　75%　in　the　hard　spot　region　in　which　the　hardness　is　about　34.　8%　higher　than　the　baseline.　Magnetic　imaging　of　pipeline　steel　of　X70　is　realized　by　using　the　parameter　of　Up　and　the　hard　spots　with　a　width　around　10　mm　is　accurately　identified　from　the　imaging　results.　The　above　research　results　provide　support　in　method　and　instrument　for　on-line,　nondestructive　testing　of　hard　spots　in　high-end　steel　plates.　©　2023　Science　Press.　All　rights　reserved.