Based　on　a　typically　simply-supported　girder　bridge　in　Wenchuan,　a　set　of　finite　element　model　samples　of　bridges　were　provided　by　considering　the　uncertainty　of　bridge　parameters.　Nonlinear　dynamic　transient　analysis　was　preformed　using　the　Wenchuan　earthquake　ground　motion　record,in　which　seismic　peak　responses　of　bridge　components　were　included.　The　seismic　demands　of　bridge　components　were　analyzed　by　principal　component　method　for　reduction　of　dimensions,　and　the　composite　seismic　demand　of　bridge　system　was　obtained.　By　using　the　regression　analysis　method,　a　probabilistic　relationship　between　the　ground　motion　intensity　and　the　composite　seismic　demand　was　established.　On　this　basis,　the　different　ground　motion　intensity　measures　were　suggested　and　compared.　The　results　show　that　the　velocity-based　ground　motion　intensity　measures　have　preferable　efficiency,　proficiency　and　sufficiency,　and　could　be　used　for　the　seismic　vulnerability　analysis　and　seismic　risk　assessment　further.