The　heavy-duty　machine　tool　is　usually　assumed　in　the　concrete　foundation,　in　which　the　machine　tool-foundation　joints　have　a　critical　effect　on　the　working　accuracy　and　life　of　heavy-duty　machine　tool.　This　paper　proposed　a　novel　contact　stiffness　model　of　concrete-steel　joint　based　on　the　fractal　theory.　The　topography　of　contact　surface　exist　in　concrete-steel　joint　has　a　fractal　feature　and　can　be　described　by　fractal　parameters.　Asperities　are　considered　as　elastic,　plastic　deformation　in　micro-scale.　However,　the　asperities　of　concrete　surface　will　be　crushed　when　the　stress　is　larger　than　their　yield　limit.　Then,　the　force　balance　of　contact　surfaces　will　be　broken.　Here,　an　iteration　model　is　proposed　to　describe　the　contact　state　of　concrete-steel　joint.　Because　the　contact　asperities　cover　a　very　small　proportion　(less　than　1%),　the　load　on　crushed　asperities　is　assumed　carried　by　other　no　contact　asperities.　This　process　will　be　repeated　again　and　again　until　the　crushed　asperities　are　not　being　produced　under　external　load.　After　that,　the　real　contact　area,　contact　stiffness　of　the　concrete-steel　joint　can　be　calculated　by　integrating　the　asperities　of　contact　surfaces.　Nonlinear　relationships　between　contact　stiffness　and　load,　fractal　roughness　parameter　G,　fractal　dimension　D　can　be　revealed　based　on　the　presented　model.　An　experimental　setup　with　concrete-　steel　test-specimens　is　designed　to　validate　the　proposed　model.　Results　indicate　that　the　theoretical　vibration　mode　shapes　agree　well　with　the　experimental　variation　mode　shapes.　The　errors　between　theoretical　and　experimental　natural　frequencies　are　less　than　4.18%.　The　presented　model　can　be　used　to　predict　the　contact　stiffness　of　concrete-steel　joint,　which　will　provide　a　theoretical　basis　for　optimizing　the　connection　characteristic　of　machine　tool-concrete　foundation.