The　seismic　damage　assessment　method　based　on　frequency　change　rate　is　applied　with　the　advantages　such　as　clear　mechanism　and　high　precision,　but　the　time　resolution　and　the　frequency　resolution　cannot　be　accurately　meet　at　the　same　time　when　the　traditional　methods　for　signal　analysis　are　used.　Thus,　the　time-varying　frequencies　cannot　be　obtained　directly　from　the　response　signals　and　the　application　of　this　damage　assessment　method　is　insufficient.　The　corresponding　accuracy　evaluation　criterion　for　different　signal　analysis　methods　is　presented　based　on　the　marginal　condition　in　time-frequency　domain,　and　wavelet　packet　decomposition　method　with　special　wavelet　basis　functions　is　verified　as　an　efficient　tool　in　establishing　time-varying　power　spectrum　on　the　basis　of　theoretical　and　computational　analysis.　The　time-varying　frequency　extraction　method　based　on　wavelet　packet　ridge　is　proposed,　and　the　time-varying　seismic　damage　index　can　be　calculated　according　to　frequency　changes,　furthermore,　the　multi-dimensional　seismic　damage　assessment　can　be　realized.　The　numerical　examples　and　experimental　results　show　that　the　damage　assessment　method　based　on　wavelet　packet　transform　and　time-varying　frequency　can　accurately　reveal　the　integral　damage　evolution　process　and　the　damage　states.　This　method　only　requires　structural　response　histories,　and　has　superior　adaptability　on　structural　dynamic　analysis,　seismic　design　verification　and　seismic　damage　assessment　for　actual　structures.