The　equilibrium　solidified　phase　diagrams　of　high　boron　high　speed　steel　have　been　calculated　and　the　vertical　section　of　iron-carbon　pseudo-binary　phase　diagrams　has　been　drawn　with　different　aluminum　concentration.　The　effect　of　aluminum　on　phase　diagrams　and　solidification　microstructure　has　been　investigated　by　using　optical　microscope,　scanning　electron　microscopy,　X-ray　diffraction,　and　differential　scanning　calorimetry.　The　results　show　that　the　austenite　region　shrinks　to　a　small　area　and　the　delta-iron　changes　into　alpha-iron　directly　during　cooling　process　when　the　aluminum　content　reaches　1.5　wt.%.　The　addition　of　excessive　amount　of　aluminum　favors　the　formation　of　ferrite,　which　leads　to　the　hardness　decreasing.　Moreover,　excessive　amount　of　aluminum　(Al　＞=　1.5　wt.%)　will　make　network　M2B　borocarbides　tend　to　break.　Alloying　with　aluminum　raises　the　solubility　of　carbon　in　the　matrix　and　reduces　the　quenched　hardness.　The　calculation　results　are　agreed　with　the　ones　from　experimental.　The　calculation　of　phase　diagrams　method　has　been　successfully　used　for　the　computation　of　phase　equilibrium　in　the　multi-component　high　boron　high-speed　steel　system.　The　work　provides　a　practical　method　for　engineers　and　researchers　in　related　areas.