There　are　many　bolted　structures　in　heavy-duty　gantry　machine　tools,　and　the　structural　performance　of　these　components　greatly　impacts　the　dynamic　properties　of　the　entire　machine.　Firstly,　based　on　the　fractal　theory,　a　bolt　preload-contact　stiffness　and　damping　model　was　established　by　characterizing　the　load,　deformation　and　strain　energies　of　the　bolted　joint　surfaces.　In　order　to　precisely　forecast　the　dynamic　features,　a　dynamic　response　model　of　the　tip　point　of　the　gantry　machine　was　first　proposed　by　adopting　the　multi-body　system　transfer　matrix　method.　The　feasibility　of　the　contact　mechanism　and　the　dynamic　response　model　were　verified　by　modal　tests　respectively.　Finally,　the　effect　of　bolt　preload　loosening　on　the　dynamic　response　of　the　whole　machine　is　analyzed.　In　the　scaled　model　of　the　gantry　machine,　when　the　bolt　preloads　on　the　lathe　bed-column　and　crossing　beam-column　joint　surfaces　are　loosened　by　30　%,　the　displacement　response　of　the　tool　tip　point　increases　by　41.2　%　and　40.1　%　respectively,　which　indicates　that　the　effect　of　lathe　bed-column　interface　is　more　significant.　This　paper　seeks　out　the　weak　link　in　the　gantry　machine　tool,　which　helps　to　carry　out　the　design　optimization　of　its　assembly　processing　afterward.