In　this　paper,　we　propose　a　new　method　to　realize　free-form　keyword　search　over　a　relational　database.　The　basic　idea　of　the　method　is　based　on　a　knowledge　base　that　stores　the　related　information　of　tuples　in　the　database.　Using　the　saved　knowledge,　we　create　an　index　and　improve　the　classic　ranking　strategy　in　Information　Retrieval;　then　the　top-N　answers　will　be　returned　for　a　keywords　query　by　the　index　and　improved　ranking　strategy.　Experimental　results　on　a　large　real　dataset　indicate　that　the　performance　of　the　method　is　efficient　and　effective.