The　frequency　content　of　earthquake　ground　motion　is　one　of　the　important　factors　which　affect　the　dynamic　response　of　structural　systems.　Because　of　the　uncertainty　of　characteristic　period　Tg　and　amplification　coefficient　?,　the　results　of　seismic　response　under　the　ground　motions　will　be　affected　by　its　uncertainty.　So　the　research　on　this　topic　becomes　so　necessary.　Based　on　the　ground　motions　during　the　2008　Wenchuan　earthquake,　the　uncertainty　of　Tg　and　?　is　analyzed　and　the　maximum　value,　the　minimum　value,　the　mean　value,　the　standard　deviation　and　the　coefficient　variation　are　presented　finally.