In　order　to　use　the　vibration　method　to　measure　the　cable　tension　of　the　external　prestressed　cable　of　Hongtan　Bridge　on　316　national　road,　according　to　the　simple　taut　string　theory　and　the　transverse　vibration　theory　of　beam,　this　paper　establishes　the　calculation　model　of　cable　tension　of　the　grouting　external　prestressed　cable.　This　calculation　model　has　two　indeterminate　coefficients　λ1,　and　λ2.　Using　the　model　testing　data　of　Hongtan　Bridge　external　prestressed　cable,　by　the　least　square　technique,　the　coefficientλ2　andλ2　are　found　out.　Error　analysis　indicates　that　the　error　of　the　formula　to　estimate　cable　tension　of　the　grouting　external　prestressed　is　small.　And　this　formula　can　be　used　to　calculate　the　cable　tension　of　Hongtan　Bridge　and　its　long　-　term　health　monitoring.