A　damage　diagnosis　method　based　on　axial　vibration　was　presented　in　the　light　of　theoretically　derived　axial　vibration　differential　equation.　The　method　takes　the　second　derivative　of　low-level　axial　vibration　modal　shape　as　damage　index.　Modal　experiment　of　axial　vibration　was　performed　on　steel　components　with　sine　wave　excitation　and　acceleration　response　collection.　The　resonance　point　was　determined,　and　the　axial　vibration　damage　index　was　calculated　based　on　the　acceleration　value　at　the　resonance　point.　The　test　and　calculation　results　show　the　damage　index　is　very　sensitive　to　both　the　location　and　degree　of　structural　damage　and　is　helpful　to　plan　the　calibration.　The　index　will　change　to　the　opposite　direction　suddenly　at　the　damage　position　and　the　magnitude　of　sudden　change　may　indicate　the　degree　of　damage.