In　the　light　of　the　shortcomings　of　the　calculation　method　for　the　diagonal　crack　width,　a　new　method　based　on　the　bond-slip　theory　is　proposed.　In　view　of　it　is　difficult　to　measure　the　external　load　or　to　predict　the　crack　width　accurately　in　actual　earthquake　scenario,　it　is　difficult　to　evaluate　the　damage　degree　of　seismic-damage　column,　the　width　calculation　method　of　transverse　cracks　and　diagonal　cracks　based　on　the　flexural　deformation　and　shear　deformation　is　established.　On　this　basis,　a　method　and　process　for　calculating　the　maximum　crack　width　based　on　the　horizontal　displacement　of　the　seismic-damaged　RC　column　is　proposed,　and　the　analytical　solution　of　the　damage　index　of　the　RC　column　based　on　the　crack　width　is　obtained.　The　analysis　of　an　example　proves　that　the　new　method　can　be　used　to　accurately　calculate　the　crack　width　based　on　the　horizontal　displacement　of　the　RC　column,　and　the　dynamic　evolution　process　of　the　crack　width　and　its　failure　mode　can　be　revealed.　In　addition,　the　example　analysis　also　proves　that　the　damage　value　of　the　RC　column　obtained　from　the　analytical　solution　of　the　damage　index　is　reasonable　and　credible.　This　study　can　provide　strong　support　for　the　rapid　assessment　of　the　seismic-damaged　RC　columns.