Heavy　duty　and　ultra-heavy　duty　machine　tools　are　used　to　manufacture　large　parts　or　super　large　parts　in　aerospace,　ship,　transportation,　and　energy　industries.　The　weight　of　a　structural　part　of　ultra-heavy　duty　machine　tools　will　reach　more　than　100　tons,　because　of　which　both　manufacturing　and　transportation　become　very　difficult.　In　this　paper,　a　detachable　design　method　for　large-sized　structures　of　heavy　duty　machine　tools　is　presented.　The　proposed　method　aims　to　make　large-sized　structure　into　several　detachable　sections　that　can　be　bolted　together　in　order　to　be　casted　and　transported　and　remanufactured　more　easily.　To　analyze　the　influence　of　the　joint　surface,　a　three-dimensional　fractal　contact　model　based　on　the　influence　of　domain　expansion　factor　is　used　to　identify　stiffness　and　damping　of　the　joint　surface.　On　the　basis　of　the　analysis　of　the　dependence　of　contact　stiffness　and　damping　of　joint　surface　on　dynamic　characteristics　of　structural　part,　a　detachable　optimization　model　of　a　super　span　beam　is　established,　and　the　particle　swarm　optimization　algorithm　is　adopted　to　carry　out　the　optimization.　After　that,　the　dynamic　characteristics　of　the　optimized　design　is　analyzed　and　verified　by　finite　element　analysis.　Based　on　the　simulated　verification,　a　detachable　beam　of　heavy　duty　gantry　machine　was　designed　by　the　proposed　method　and　verified　by　the　field　test.　The　illustration　example　shows　that　the　large　span　beam　can　be　detached　into　three　sections　that　can　be　bolted　together　by　high　strength　bolts,　and　it　also　has　satisfied　performance.