The　microstructure　of　Al-bearing　high-boron　high　speed　steel　(AB-HSS)　contains　a　large　amount　of　borocarbides,　which　makes　it　have　high　hardness,　superior　wear　resistance　and　thermal　stability.　But　the　borocarbides　are　coarser　and　continuously　distribute　along　the　grain　boundary　which　seriously　destroys　the　toughness　of　AB-HSS.　In　this　paper,　the　microstructure　and　properties　of　AB-HSS　were　regulated　by　adding　modifiers　and　quenching　and　tempering　heat-treatment.　The　modifier　was　RE-Mg,　Ti　and　N　elements.　The　microstructure　and　properties　of　AB-HSS　were　investigated　by　means　of　optical　microscopy　(OM),　scanning　electron　microscopy　(SEM),　X-ray　diffraction　(XRD),　the　electron　probe　microanalysis　(EPMA),　hardness　testing,　wear　testing,　and　impact　testing.　The　results　show　that　the　borocarbides　in　as-cast　AB-HSS　are　found　in　intergranular　networks　showing　different　degrees　of　continuity　after　adding　different　types　of　modifiers.　The　matrix　of　as-cast　AB-HSS　all　is　composed　of　ferrite,　pearlite　and　martensite,　and　the　borocarbides　all　consist　of　M-2(B,C)　and　M-7(C,B)(3)　after　adding　different　types　of　modifiers.　After　high-temperature　heat　treatment,　the　borocarbides　in　the　microstructure　of　RE-Mg　modification　AB-HSS　appear　as　discontinuous　networks　showing　signs　of　spheroidization　and　the　distribution　of　the　borocarbides　is　more　uniform.　The　matrix　of　RE-Mg　modification　AB-HSS　is　martensite　and　the　borocarbides　consist　of　M-2(B,C),　M-7(C,B)(3)　and　M-23(C,B)(6).　The　hardness　of　RE-Mg　modification　AB-HSS　reaches　61.7　HRC,　and　impact　toughness　increases　by　52%,　and　has　excellent　comprehensive　mechanical　properties.