Water　permeability　in　concrete　is　one　of　the　main　reasons　behind　deterioration　of　concrete.　Hydraulic　pressure　method　which　is　designed　according　to　Darcy＇s　law　is　the　leading　method　to　evaluate　water　permeability　in　concrete　at　present.　However,　with　the　development　of　concrete　material　and　construction　technology,　the　porosity　of　concrete　reduces　and　density　of　concrete　increases　gradually,　hydraulic　pressure　method　loses　applicability　for　evaluating　water　permeability.　Water　absorption　test　is　designed　based　on　transport　theory,　and　computational　model　of　diffusion　coefficient　is　gained　by　perform　mathematical　calculations.　The　results　indicate　that　the　computational　diffusion　coefficient　could　be　used　to　evaluate　water　permeability　in　mortar　or　concrete.