Bolted　joints　are　a　critical　fastening　method　widely　used　in　mechanical　structures,　and　monitoring　their　loosening,　particularly　in　the　early　loosening,　is　crucial　for　ensuring　structural　safety　and　reliability.　This　article　presents　a　method　for　full　lifecycle　quantitative　monitoring　and　early　warning　of　bolt　loosening　based　on　acoustic　emission　(AE)　technology.　Variational　mode　decomposition　is　applied　to　the　AE　signals　from　the　bolted　joint　interface　to　extract　the　energy　characteristics　of　the　intrinsic　mode　functions　(IMFs).　The　IMFs　most　sensitive　to　changes　in　preload　force　are　then　selected　as　the　characteristic　mode.　Segmented　fitting　equations　are　developed　for　the　relationship　between　the　energy　of　this　characteristic　mode　and　the　bolt　preload　force　across　three　key　stages　of　loosening:　early　loosening,　intermediate　loosening,　and　late　loosening.　These　equations　enable　effective　monitoring　and　quantitative　assessment　of　bolt　loosening.　This　method　not　only　allows　for　full　lifecycle　monitoring　of　bolted　joints　from　fastening　to　loosening　but　also　provides　effective　early　warnings　during　the　initial　loosening　phase.