In　this　article,　the　maximal　runoff　of　some　section　of　road　under　rainstorm　condition　is　computed,　the　correlative　expression　of　porous　asphalt1　mixture＇s　permeability　coefficient　and　void　is　established　through　its　indoor　permeability　test,　what＇s　more,　porous　asphalt　pavement＇s　objective　void　that　reducing　the　maximal　runoff　requires　is　calculated.　It　is　validated　that　the　porous　asphalt　mixture　sample　applied　this　objective　void　satisfied　the　needs　of　mechanics　index　under　traveling　condition,　so　this　porous　asphalt　pavement　can　reduce　urban　flood　induced　by　rainstorm.　Copyright　ASCE　2009.