The　goal　of　semantic　search　is　to　abandon　the　search　model　that　based　on　simple　keyword　matching,　combined　with　semantic　technologies,　make　full　use　of　the　characteristics　of　ontology-based　semantic　information　relationship　to　provide　users　with　more　high-quality　and　efficient　search　results.　The　study　of　this　paper　focuses　on　the　semantic　extension　of　keywords　in　semantic　search　process,　to　consider　the　vocabulary　expansion　with　different　factors,　thus　providing　a　set　of　factors　that　affecting　the　expansion　of　the　vocabulary　and　the　reasonable　weight　for　these　factors　that　verified　by　experiment.