The　structural　seismic　damage　assessment　method　based　on　frequency　changes　has　the　advantages,　such　as,　clear　mechanism　and　high　precision,　but　the　time-frequency　resolution　for　the　traditional　methods　of　signal　analysis　can　not　accurately　meet　the　requirements　of　precision.　Hence,　the　time-varying　frequencies　cannot　be　obtained　directly　from　the　response　signals　and　the　application　of　this　damage　assessment　method　is　affected.　Here,　accuracy　evaluation　criteria　for　different　signal　analysis　methods　were　presented　based　on　marginal　conditions　in　time-frequency　domain,　and　the　wavelet　packet　decomposition　method　with　special　wavelet　basis　functions　was　verified　as　an　efficient　tool　to　establish　time-varying　power　spectrum　on　the　basis　of　theoretical　and　computational　analyses.　The　time-varying　frequency　extraction　method　based　on　wavelet　packet　ridge　was　proposed,　and　the　structural　time-varying　seismic　damage　index　was　calculated　according　to　structural　frequency　changes,　furthermore,　the　structural　multi-dimensional　seismic　damage　assessment　was　realized.　The　examples　showed　that　the　seismic　damage　assessment　method　based　on　wavelet　packet　transformation　and　time-varying　frequencies　can　accurately　reveal　the　overall　damage　evolution　process　and　the　eventual　damage　level　of　a　structure;　this　method　only　requires　structural　displacement　histories,　and　has　a　good　applicability　for　structural　dynamic　analysis,　seismic　design　verification　and　seismic　damage　assessment　of　actual　structures.　©　2016,　Editorial　Office　of　Journal　of　Vibration　and　Shock.　All　right　reserved.