Damage　detection　methods　based　on　natural　frequencies,　vibration　modal　shapes　and　other　basic　dynamic　parameters　are　deficient,　their　accuracy　and　applicability　should　be　improved.　Here,　damping　ratio　was　proposed　as　an　indicator　for　damage　detection　to　explore　the　relationship　between　damping　and　structural　damage.　The　relationships　among　damping,　number　and　width　of　structural　cracks　and　damage　evolution　were　described　theoretically.　Through　a　simply　supported　T　beam　bridge　test,　the　free　vibration　method　was　used　to　calculate　the　bridge　system＇s　damping　ratios.　The　system＇s　damping　ratios　under　different　damage　levels　were　obtained.　The　results　showed　that　the　system＇s　damping　ratios　firstly　increase　and　then　decrease　with　increase　in　damage　levels;　the　difference　between　damping　ratios　corresponding　to　positive　displacement　and　negative　one,　respectively　has　the　same　evolution　law;　the　location　with　a　larger　changing　rate　of　damping　ratio　may　have　a　more　serious　damage,　so　the　changing　rate　of　damping　ratio　can　be　used　to　realize　locating　basic　damages;　compared　with　natural　frequencies　and　vibration　modal　shapes　of　bridges,　damping　ratios　are　more　sensitive　and　vivid,　so　they　can　be　taken　as　indicators　of　damage　detection　for　RC　bridges,　and　their　changing　inflection　points　can　be　taken　as　early　warning　thresholds　of　structural　reinforcement　and　maintenance.　©　2017,　Editorial　Office　of　Journal　of　Vibration　and　Shock.　All　right　reserved.