A　new　method　taking　the　second　derivative　of　low-level　axial　vibration　modal　shape　as　damage　index　is　presented　on　the　basis　of　theoretical　derivation　axial　vibration　differential　equation　and　analytical　solution　of　three　kinds　of　support　form.　The　index　is　very　sensitive　to　damage　location　and　damage　degree.　In　addition,　the　effect　of　point　number　is　analyzed　and　results　showed　that　with　a　few　points　can　still　calibration　damage　well.