Modal　parameters　are　inherent　characteristics　of　civil　structures.　Due　to　the　effect　of　environmental　factors　and　ambient　loads,　the　physical　and　modal　characteristics　of　a　structure　tend　to　change　over　time.　Therefore,　the　effective　identification　of　time-varying　modal　parameters　has　become　an　essential　topic.　In　this　study,　an　instantaneous　modal　identification　method　based　on　an　adaptive　chirp　mode　decomposition　(ACMD)　technique　was　proposed.　The　ACMD　technique　is　highly　adaptable　and　can　accurately　estimate　the　instantaneous　frequencies　of　a　structure.　However,　it　is　important　to　highlight　that　an　initial　frequency　value　must　be　selected　beforehand　in　ACMD.　If　the　initial　frequency　is　set　incorrectly,　the　resulting　instantaneous　frequencies　may　lack　accuracy.　To　address　the　aforementioned　problem,　the　Welch　power　spectrum　was　initially　developed　to　extract　a　high-resolution　time–frequency　distribution　from　the　measured　signals.　Subsequently,　the　time–frequency　ridge　was　identified　based　on　the　maximum　energy　position　in　the　time–frequency　distribution　plot,　with　the　frequencies　associated　with　the　time–frequency　ridge　serving　as　the　initial　frequencies.　Based　on　the　initial　frequencies,　the　measured　signals　with　multiple　degrees　of　freedom　could　be　decomposed　into　individual　time-varying　components　with　a　single　degree　of　freedom.　Following　that,　the　instantaneous　frequencies　of　each　time-varying　component　could　be　calculated　directly.　Subsequently,　a　sliding　window　principal　component　analysis　(PCA)　method　was　introduced　to　derive　instantaneous　mode　shapes.　Finally,　vibration　data　collected　under　various　operational　scenarios　were　used　to　validate　the　proposed　method.　The　results　demonstrated　the　effective　identification　of　time-varying　modal　parameters　in　real-world　civil　structures,　without　missing　modes.　©　2024　by　the　authors.