The　separation　performance　of　mixed　matrix　membranes　(MMMs)　relies　on　their　microstructures　and　composing　materials.　Metal-organic　frameworks　(MOFs)　are　a　class　of　promising　materials　as　porous　fillers　of　composite　membranes.　However,　high　quality　MOF-based　MMMs　for　water　treatment　are　still　in　lack.　In　this　study,　highly　hydrolytic　stable　MOF　crystallites　of　BUT-8(A)　(BUT-8(A)　=　Cr-3　(mu(3)-O)　(H2O)(3)(NDC(SO3H5/6)(2))(3),　NDC(SO3H)(2)(2-)=　4,8-disulfonaphthalene-2,6-dicarboxylate)　in　size　of　about　30　nm　were　incorporated　into　polyethyleneimine　(PEI)　matrix.　Benefited　from　the　excellent　dispersibility　of　BUT-8(A)　in　water　and　the　good　compatibility　between　the　MOF　and　polymer,　a　thin　and　uniform　active　BUT-8(A)/PEI　layer　with　a　high　MOF　loading　(50　wt%)　was　obtained　on　a　hydrolyzed　polyacrylonitrile　(HPAN)　substrate.　The　BUT-8(A)/PEI-HPAN-50　composite　membrane　exhibited　high　water　permeances　(396-683　L　m(-2)　h(-1)　MPa-1)　and　high　rejection　rates　for　aqueous　dye　solutions.　In　addition,　the　separation　mechanism　of　resulting　membrane　was　explored　and　the　high　water　permeance　of　membrane　was　attributed　to　the　facilitated　molecule　transportation　through　the　channels　of　BUT-8(A)　rather　than　the　defects　between　MOF　and　polymer.　The　high　separation　performance　of　resulting　membrane　suggests　a　promising　prospect　of　MOF　based　composite　membrane　for　water　purification.